Why is murder wrong if Many Worlds Interpretation of Quantum Mechanics is true?

[Aegon]

I want my father dead. So, what is wrong with me making him dead for me? I am not making him dead for himself.

Whether you're technically killing them based on physics does not really have much bearing on the ethics of killing someone generally, IMO. Because even if QI is true, one's sense of self is relative and is constructed over the course of one's life. You may not technically be ending their existence entirely, but you are absolutely prematurely ending the life of this version of them and the real sense of self that they experience. I don't think the ethics of murder change one bit under these circumstances.