How can someone who doesn't exist commit murder?
You are trying to explain something
but it cannot be murder. Even if we overlook the fact that someone who does not exist at the point of their 'death' cannot commit murder, the law permits you to kill others in self-defence
. If those were three random chracters and Tuvix was walking the street and was confronted by Tuvok and Neelix and if he did not kill them, then they would kill him - then he would be able to lawfully kill them.