Reiner truly has it bad in AOT image getting brainwashed in thinking everyone on paradis island is a devil only to go there and find out yourself that it just isn’t true. He built stable relationships with them and realised that they’re just like him. They all want to be hero’s-
And to save their people. Reiner realised that any decisions he makes will result in many deaths and will hurt him emotionally. He wants to save his own people but he knows the only way to do this is to kill everyone from paradis. It’s hard to kill people you built bonds with-
So he decided he doesn’t want to make a decision, he doesn’t want any more pain, suffering and to see more deaths. The only way out for him was death but in the end he couldn’t end his own life so he even begged Eren to end it for him
He wants a peaceful death but how can you ignore the pleading cries of children who look up to you? This showcases Reiner’s humanity (his titan form is more human too) he fights not because he wants too but because if he doesn’t everyone he loves will die