Final Bladerunner thought:

Sometimes there are characters that act irrationally becuase they’re basically sociopaths. They kill without consideration.

My problem with Roy is that he was written that way .... right up until the end, and nothing happened to explain a change.

Either Roy has some motive to kill Sebastian that was poorly articulated (eg badly written and plotted) or he didn’t. It would make sense for Roy to resent Sebastian for being complicit in the subjugation of replicants — except it’s was explicitly written as sympathetic...

...becuase he too was rejected by humans due to his disease.

If you give him a backstory that makes him sympathetic to replicants, you should explain why that backstory doesn’t move Roy to let him live, but Roy IS moved to spare Deckhert— a replicant killing machine.