I'll assume you didn't mean to imply that Tuvix was less worthy of existence simply because he wanted to live instead of committing suicide in a procedure that -might- restore his constituent parts. To me, it's pretty cold-blooded to not have -some- sympathy for a creature wanting to preserve its own existence.
Well, no, I think most of us do feel sympathy for Tuvix. But we also feel it for Tuvok and Neelix as well. Why should Tuvix's right to life (if he has any) outweigh theirs?
Do the people who feel killing Tuvix was the right move also believe that the Baku should have been relocated?
No, since the Ba'ku were native to that planet and had every right to be there. Even if they weren't originally born on that planet, the Federation had nothing to do with their being there (unlike Tuvix, who exists only because of a transporter accident that occurred on a Starfleet ship), and thus the Feds have no business interfering.
That Qunn should have been destroyed because his continuing existence represented a danger to the Q continuum?
Quinn's fate was decided by entirely legal means. He requested asylum, the Q demanded a hearing, which they lost. They abided by that decision. There was never any problem after that.
If you could go back in time and kill Khan, Hitler, etc. at birth, would you?
No. For two reasons: 1) I'm not sanguine about killing babies, and 2) Interfering with history is always wrong, because you never know if the new timeline might be even worse.