The creature says he is evil because he is unloved. Is the treatment he has received from people an acceptable excuse for his crimes? Why or why not?

This is a pretty subjective question. Can someone be pushed too far before they break and wreak havoc on the world? We saw this same narrative in Todd Phillips The Joker (2019) where Joaquin Phoenix asks this same question. I really think this is a morality question which is up to your opinion.