I've come to love this scene much more after reading the latest newsletter because it demonstrates how Asriel's relationship/understanding towards Chara changed for the better after the Pacifist ending.
It shows that even after saying that Chara "wasn't the greatest person", Asriel doesn't completely hate or resent Chara despite their failures and shortcomings as siblings, and he still wants to see them happy and content.
When Asriel said Chara "wasn't the greatest person," it meant that he let go of his unhealthy view/relationship with them; NOT THAT HE LET GO OF THE LOVE AND CARE HE STILL FEELS FOR THEM AS HE STILL WANTS TO HAVE A RELATIONSHIP WITH THEM!
Another way this scene greatly demonstrates Asriel's growth as a character is that it also shows that despite everything, Asriel/Flowey still recognizes Chara's CAPACITY FOR GOOD DESPITE THEIR MANY FLAWS.
The line "nothing can hurt anyone anymore" PROVES that he isn't saying that because of wishful thinking, but that he KNOWS FOR A FACT that Chara had good intentions (even if they paved the road to hell with them).
So when Flowey tells Chara there's nothing to worry about, he's reassuring/comforting Chara by telling them that they finally got what they wanted and that the people they care about have got the freedom and prosperity that Chara wanted for them all along, even if it didn't happen the way they intended!
