Hmm...I struggle with this myself and you sound like me.

Sometimes I try and look for something good I have done in my life. Maybe even something as "small" as holding the door for an elderly person....then I try and find other things and make a list.

I then have a list and I look at the list and try and tell myself that a "truely horrible person who should be killed" would not have done these things.

Or I try and break my life into segments. And look to see if over these segments I have done things and grown. A "truly horrible person who should be killed" does not grow positively over time.

Another method I use is to compare my actions to those actions of people that hurt me in my life. Sometimes just realizing how different my actions have been compared with theirs helps challenge beliefs about myself.

It is a struggle everyday, but try looking for small things that challenge those thoughts.