Interpretation

I was watching a movie and I came across this: A: You are not needed, welcome, or wanted. You have only been spared because you're my brother's greatest weakness. And I cannot show mercy to those who would demonstrate threat towards this family.

If anyone needs any context, the person (B) who has been spared was sent to a jail instead of being sentenced to death (im pretty sure).

How is it "not showing mercy" by sparing someone?

Can someone tell me their interpretation, and what I could be missing?