*Everyone giving advice to Arthur*
Gwen: It's okay to ask for help.
Lancelot: You're not a burden.
Merlin: Murder is okay.
Leon: Your feelings matter.
The funny thing is - to me- that Merlin would say that to himself, but not actually to Arthur. Because Merlin believes in Arthur's good heart and keeps telling him to not kill people because "what would that achieve" with his hands bloody and a knife already in the back of Arthur's enemies.
What's even funnier - is I believe out of all the characters you named, the one I'd apply with 'murder is okay' is, ironically, Gwen. Because she herself wouldn't do it but she'd understand that Arthur has to do it for the Kingdom.
That said, ignore me, OP is fucking funny.