Imagine: I'm a mid-level fighter, retired and living in a peaceful village. News starts coming in of a dragon terrorising the area. It's burned three other villages so far. Then a dragon flies over and lands, slap bang in the middle of my village.
Now, my most prized possession is one (1) Arrow of Dragon Slaying. 100% guaranteed to kill, stone dead, any dragon it hits.
Is this the same dragon that's been terrorising others? Or is it a completely different dragon, here on an embassy of peace? There's no way to tell. What I do know is that right at this moment, my family, everyone I love and care about, is one sneeze away from death.
So, is this specific dragon "evil"? I don't know, and right at this moment I don't care. I'm going to take the shot.
Is that good, is it morally defensible? Again, I don't care. Whatever the consequences are, I'll deal with them as best I can. Right here and now, there is an immediate and obvious threat, and I have to make a choice: do I finish it, or do I wait to see what it does next? Sorry, dragon, I can't take that chance.
You want to call me a monster for that? Fine, say it with me now: I don't care. I'll even come to your families' funerals, when it's their turn.