very topical question. i was just thinking about this, actually.
it's tempting to think Sister Imperator simply hated Terzo and couldn't wait to get rid of him so she could put Cardi in charge, which i think is at least a tiny bit true, but not entirely true.
i watched the Era 3 lore video series 'The Summoning' with some friends the a while ago, and it was the first time they'd watched it. one of them remarked they were surprised that Sister Imperator seemed to be genuinely hyping up Terzo.
this is true! Sister Imperator did actually hype up Terzo during his first year as Papa. at the beginning of Era 3, she was deeply disappointed with the Nameless Ghouls and The Ghost Project's failure to make significant progress in their mission to convert the world to their cult. she was initially very excited for Terzo's reign and believed that he would be extremely successful.
SISTER IMPERATOR: Brothers and Sisters, you know why you are here tonight. This Ministry is now seven years into The Ghost Project. Seven long years of work. Two Papas, two albums, one gold. These are indeed some respectable numbers, but let me give you some others. Churches opened: zero. Governments toppled: zero. World leaders converted to the cause: zero. You call yourselves salesmen? Masterminds? You have done shit! And don't blame the music. The music is the very manifestation of His Dark Majesty. And don't blame this fine merchandise! It's a disgrace! Papa 2 has been let go. He is a miserable, wounded, and bitter old man, and he is washed up! But let's look forward now. This is a new piece of music. And these are your new masks. And this is Papa 2's brother. He is a full 3 months younger. This man will take the band further than any of us could ever have imagined. I present to you now… Papa Emeritus III!
The Summoning (part 1) (May 20, 2015)
Terzo did end up being very successful, even before Meliora released.
SISTER IMPERATOR: It has arrived! Some of you did not believe the new Papa would be able to cast a shadow as wide and as far as his great predecessor. Your faithlessness is now dust in my mouth. You should be on your knees humbling yourselves before what has been accomplished here! All you need to do is look outside. Look at the graffiti on the walls, look at the lights illuminated at night, where once there was only darkness.
The Summoning IV: The Arrival || Unholy / Unplugged - Los Angeles, California, USA (August 21, 2015)
Sister Imperator felt she was right to have faith in Terzo. she continued to preach about Terzo's power and demanded that others respect him.
SISTER IMPERATOR: Welcome! Welcome, my faithful brothers and sisters! Your presence here is proof of your commitment. If you are unsure, cast off your doubts now. There is no turning back. The rite you're about to witness is but one small but essential movement in our spiritual revolution. We prayed, and he has arrived! But! But! He will demand more of you! He will need to hear from the abyss of your hearts that you are ready. His is a voice of the pit and the pinnacle! His Nameless Ghouls are the music of the [UNINTELLIGIBLE], but your holy noise is the key! You must cry out his name! Say it with me now: Papa Emeritus! Again! Again! Again! Very good. Shh! Shh! Shhhhhh! Listen. Do you hear it? Do you? It's the terrible sound of the ignorant– the mistrust and anger of the masses. The world is unstable and they have lost their balance. But we, here together, are the new foundation. We are the shape of things to come! There's not much time. We won't be able to do the required incantation. We must let the music do the summoning. My brethren, my brethren, bow your heads and raise your horns to pierce the veil of heaven, so the skies will be torn asunder! And Papa may fall into our midst! Now is the moment. Now, there is no other. Papa Emeritus III! Ghost! Ghost is here!
Los Angeles, CA, USA (October 26, 2015)
so what was the turning point? the 2016 Grammy win.
listen. Sister Imperator hated that Grammy so fucking much. this seems weird because she wanted the band to be successful, but here's the thing: Sister Imperator is a zealot. a real religious freak. as shown by her words in The Summoning part 1, she fully believes that the true mission and purpose of The Ghost Project is to serve satan and convert the world to his church. she was happy that Ghost's success meant more people were hearing their message, but she was very, very mad that Terzo and the Nameless Ghouls seemed to be enjoying their success a little too much. she felt like they had lost focus, forgetting their mission for satan and instead focusing on their commercial success. and to her, the Grammy was representative of the establishment / mainstream society, something they should avoid as the leaders of satan's flock of black sheep.
SISTER IMPERATOR: The industry has noted our good works with their trinket. And as a result, our message is carried further and wider. But do we take such trinkets as sacrament and the measure of true accomplishment? No! We don't need their approval. The truth of our work is not measured by awards and nods from the establishment. [...] I have here letters from your followers, demanding that we explain why nothing has changed. They have made their own sacrifices. But what have you done? How will you answer them? By holding up your golden gramophone? Is this the change you promised? Is this the sign of a new age? It is nothing! It is another false idol.
The Summoning V: The Square And Hammer (September 13, 2016)
notably, she never said anything negative about Terzo while verbally abusing the Ghouls. she still demanded respect for him and his position.
SISTER IMPERATOR: You are supposed to lead. It is your task. Your task! To lead! I think you're afraid of real change. Let's take a breath. I think I understand the problem. You think Papa's words should be enough. But then you misunderstand the nature of true power. Papa is not a mouthpiece for the Dark Divinity. He is not a pawn. He is a mediator. He is the path. His way is the truth and the darkness! And you– you are his apostles. And yet, you deny, you deny!
The Summoning VI: The Proceedings Intensify (Oct 17, 2016)
... but that doesn't mean she wasn't also mad at him, too. we just didn't see it.
Terzo was very proud of his Grammy, and he did like to brag about it a little bit. Sister Imperator would have definitely been mad about that.
PAPA EMERITUS III: Alright! How are you feeling now? Quite good, right? Yeah! Alright, I know you like your hard-rocking shit here in Skåne, right? Enough of those ballads– award-winning ballad, actually.
AUDIENCE: [APPLAUSE]
PAPA EMERITUS III: Oh, thank you.
Malmö, Sweden (February 25, 2016)
PAPA EMERITUS III: Yes! A Grammy award winning song! Right here, right now! That doesn't happen every day.
Acoustic performance at 93X Radio - Minneapolis, Minnesota (July 28, 2016)
Sister Imperator had respect for Terzo's position as Papa, but she didn't respect him.
Sister Imperator started making plans to replace Terzo as early as November 2016, almost an entire year before he was dragged off the stage at his last concert. while i do think Sister Imperator disliked Terzo and was happy to get rid of him, i don't think she felt her decision had anything to do with her personal feelings. Sister Imperator talks directly to satan (as shown in Chapter 5) and receives visions (as stated in The Summoning part 5 and part 7). she had a vision that indicated it would soon be time to replace Papa 3 with Papa 4. she started making arrangements accordingly, and Nihil went along with it.
SISTER IMPERATOR: Despite your weakness, and your cowardliness, your failures, I still believe in you. And I believe in you because I have seen the future! I have already been witness to three transfigurations, and each time I can see that we are closer to the final glory, and I have seen you rise to the occasion each time. So can you do it again? [...] After all we have been through in these past few days, all the shames laid bare, are you ready to start anew? The new coming is about to begin. Will you take up your instruments? Will you be able to commit to the utter annihilation of all this is false? All that is greed? All that is staid and conformist and empty? Hm? Then rise. RISE!!!! Now! I give you another chance at transformation. But you must beg. You must demand to be sacrificed! You must prostrate your hearts while you stand tall in the dark, for the fourth incarnation of Papa will guide us.
The Summoning VII: Believe This (Nov 14, 2016)
i don't know why Sister Imperator and Papa Nihil decided Terzo's reign would end on September 30th, 2017. and i can't fully explain why they chose to humiliate him by having him dragged off the stage. (even Secondo, who they thought was terrible at his job, was allowed a dignified ending.) and i can't explain why they desecrated his body by using his severed head as a prop for a photoshoot. but idk, it kinda seems like maybe they just hated him.
TLDR: Sister Imperator genuinely believed in Terzo, but she turned on him when he started acting in a way she didn't like. Terzo died as he lived– being used and betrayed by the people he dedicated his life to.