Dawn herself is no threat to anyone. Especially not to the universe. She is innovent. Now what you are saying is that an innocent person can be killed if you save the life of others even if that innocent person is not threatening anyone else.
Are you really sure about that ? Just imagine someone is threatening to kill you if you do not kill - let`s say - your little sister. Are you really justified in killing your sister ? Are you justified to do so if the life of two people is threatend ? Of ten ? Of hundreds ? Where do you tip the scale ? No, Buffy woul never kill innocent people. Fighting the ones who threaten the innocent people is more her style ;-)
Dawn is a threat in and of herself, because she exists, so yes, she is a threat to the Universe. She can't protect herself, and must rely on someone else saving her so the Universe isn't sucked into hell because of her.
Is she a threat in that she'll kill anyone with her bare hands, no. But obviously, the rest of the Universe would like to stay alive as well, most people in the world are innocent (at least in terms of 'do they deserve to die if Buffy fails because Glory got to Dawn first?' no they don't, no more so than Dawn and in sheer numbers they far outweigh her) From a utalitarian point of view that scale tips where you can maxime the good over the bad, I'd say. It's not a clear cut scale, but hell dimension vs normal world seems like a pretty good scale in defense of anyone wanting to kill Dawn. I'm not saying they should've, been the "are you justified in killing"... seems like a bit of a ridiculous argument to me. Ben was technically innocent too, just like Dawn he didn't ask to be connected to Glory, and I don't see many tears shed for him or much condemnation against Giles either. Because it was good for the many.
Again I'm not saying they should've killed Dawn. But Buffy saying she'll kill them if they try just shows that she is not a great leader or champion, she's too myopic and focused on her own problems. She plays with the world hanging in the balance. Because it's her show, she gets to win and it's not a problem, however that doesn't mean she's making the right or justifiable calls. Of course, later on she becomes more about the mission and becomes rather unlikable because of it, so it makes sense that this champion Buffy is more popular. It's fiction and we get to see her win because we want to, that's a beautiful thing, the Buffyverse doesn't really deal with reality after all.
But "Dawn is innocent" holds very little traction as an argument when so many other innocents are involved in the equation.
