The main character kills people---yes, that's true. But the way he justifies it almost makes it less horrible. He only kills people that he knows are a threat to society. He sees it as his calling. He satisfies his need to kill and protects the world from criminals.
Does he ever make a mistake? Meaning, has he ever killed somebody he thinks is a criminal but is actually innocent?
I really don't think I would be turned off by a serial killer as lead.
I think I would. See above.