I'm usually one for "NO MERCY. KILL THEM ALL!", but this time, I almost get it, really.
Does he even actually say "NPC"? Also, he did his thing as hero. Literally putting his life on the line to help. And he got tossed aside, Woody style, the moment some random dudebros showed up out of nowhere.
I don't know that he's killed anyone. Also, the guy got tossed aside like an old rag the moment someone else showed up. Can you blame him for taking that personally?
True, he perhaps did not kill anyone aside from demons - at least not confirmed. It doesn't make him any less irredeemable though. According to the backstory, he was a nobody in his original world, but it doesn't seem like he got bullied or anything. He just wasn't popular/full of friends. Then he got to another world threatened by the attack of demons, and got to work with a power that at first glance did not seem to do much. He spent time there, got some praises and became the center of attention. Then other heroes appeared and quickly took care of what he had struggled with so far. The heroes also did not bully or harm him in any way. Neither did the people of this world. They literally were happy because their suffering was finally over. Then again, he came back and started doing the very same thing to other worlds: Going there, getting the DL slayed. At some point (not exactly known when) he also got to take the heroes power. Delivering judgment and punishment by himself. He looked down on others, manipulated and lied, backstabbed and seemed more than ready to kill the MC. If 99% of the heroes are assholes, he is definitely part of that 99%.
Reasons NOT to kill him:
1- Heroes are merciful
2- He strayed from the path because he got tossed aside
3- He only harmed the bad guys (?)
Reasons to kill him:
1- He took away from others for his own benefit left and right, seeking for more and more power aside from the consequences
2- He lied and deceived others in order to do so, getting them beaten and harmed near death in a vegetative state without the root of their souls, which would probably get them killed if the powers robbed did not disappear with their deaths
3- He got conceited and looked down on other heroes who shared the same fate as he did, including ones who weren't assholes but got in his way, trying to do what is right
4- He strenghtened the DL of the world so that it could beat the heroes, which also means more time alive that the DL could use to kill/harm more people before he got to it (No matter which reason for that is chosen, they're all selfish and bad)
5- He was more than ready to kill the MC if he had the chance
6- He only got some regret for his actions after all his lives where stripped, meaning that he wasn't sorry for what he did, but because he would have to pay for it, to face the thereafter.
7- He witnessed the MC using his powers, that could lead to his secret getting leaked in the future because there doesn't seem to be any kind of restriction, seal or mind control power in place refraining him from doing so.