So if someone is lost in a transporter snafu, when do you declare them dead? Their signal merged to become an entirely new signal- a new lifeform. At that moment they ceased to exist. The doctor may not have declared them dead, but since their transporter patterns were lost and their matter destroyed and reconstructed, they were biologically dead.
Not according to the EMH in the episode.
If their patterns were lost then there never was any chance at separation. According to Tuvix himself, Neelix and Tuvok were alive in him. He had their memories and experiences. He also willingly agreed upon his creation that he would give up his life if a cure was found to bring them back. He knew all time that his life wasn't promised.
Plus, if this is the case then de-Borging Picard and Seven was also killing a lifeform because they both cease to be Jean-Luc Picard and Annika Hansen and are as Borg are new life.