I personally don't think belief in an afterlife should be necessary to believe it's worthwhile to not be shitty to people.

"What goes around comes around" suffices for me.

Call it "ethics", call it "maximizing outcomes for all involved stakeholders", call it "karma", "good business", or "kindness"...whatever you call it, I don't think it's difficult to find your own personal justification for it if you want to.