This is a bit like saying that if someone thinks God doesn't exist, they might think it's OK to disobey God's laws and start killing people.
Morals are separate from metaphysics, and a moral person would never advocate for eugenics or genocide regardless of their metaphysical beliefs. I personally subscribe to the belief that human minds are computers, and yet feel no urge to replace humans with computers.
As a complete side note, I don't think replacing humans with computers would be a form of eugenics. Eugenics refers to the practice of deciding which humans have a right and/or duty to reproduce for the (supposed) betterment of the species - such as through forced sterilization of "inferior" individuals, incentives for or even compulsory breeding for "superior" individuals etc. Simply killing all humans to allow for machines to rule is more like a form of genocide.