I'm saying that the moment they figured out how to separate them, they wee OBLIGATED to separate them. Because allowing Tuvix to continue to exist would now be at Neelix and Tuvok's expense.
They didn't "kill" a sentient being, they separated an accident into the two distinct personalities from which it was made up. There was no Tuvix. There was only the result of an accident complaining that it had the "right" to exist. It didn't.
Do you understand that there was no death involved? Tuvix thought he had the right to exist as a combination of Neelix and Tuvok. Wrong. They had the right to exist as individuals, He didn't have the right to exist as an unwilling combination of the two..