Are you kidding? Intent and deed. Tuvix didn't kill Neelix and Tuvok. Tuvix didn't cause the accident. He was the result of it. Regardless of how he came into being, once he did, he was an individual with all the rights of one. The only thing he was guilty of is refusing to sacrifice his live to restore Tuvok and Neelix back to life. Which is his right as a sentient being. The Borg intend to assimilate everyone into their Collective. The Borg flat out declared freedom and self-determination are irrelevant. If you resist, they'll force you into their Collective. You're not a person, you're an object to be consumed. All the moral difference in the world.