He was, but that was Deku's ultimate test. He always tried to find some redeeming factor in his enemies, sometimes inspiring them to change themselves for the better but some people can't be saved, so what is Deku willing to do when that happens? Is he willing to defeat someone when he knows they are beyond saving and beyond redemption?
He was, but he sacrificed the thing he treasured the most for it.
And while I agree with you that he was a victim, he at least died as himself, not as fragment of himself hiding deep inside of his own body.
I don't agree with many decisions Horikoshi made in terms of the development, but I think having Shigaraki's destruction be Deku's ultimate challenge was a good call. For the first time since the story began, they saw each other eye to eye and both him and Shigaraki did the right thing. I always had the feeling the story would have a bittersweet ending, and I think Shigaraki's ending, as the last wielder of One for All was emblematic of the quirk itself. He sacrificed himself for everyone, just as Deku had throughout the time they knew each other.
Deku was not able to save Tenko, but Tenko was able to save everyone.
OFA's will was fulfilled.
Again, not everything Horikoshi did, or the pacing with which he did it were things I appreciated, but I think Shigaraki's death was well accomplished both for him and Deku.