If people have been decent to me, why would I want to be anything but decent to them? If people haven't been decent to me why would I want to emulate that? Decency is basically the only thing compatible with rational self interest.

What do I get out of ruining life for others? Is theft & destruction a fulfilling sort of pursuit? Is killing someone or in some way scarring a person for life not going to haunt me?

I find it a bit concerning when people don't seem to understand that being a good person & doing what is right (even if it's hard) is the path to building self respect & any sort of meaningful sense of satisfaction in life.