My point is not what my line for murdering someone is or how it compares to anyone else's. My point is that most people have that line, so it's not very deep to point out that it exists, and all of them will believe that they are still the Good Person and the person they want to murder is the Bad Person as laid out in the OP.

Now the very last sentence is a bit wibbly. I'm sure that many people might not automatically send someone who disagrees with their line into the Bad Person zone, but I'm sure you could construct scenarios similar to the last line that people would end up agreeing with, they just think that their particular line is justified.