First off, I have no problem with fictional characters killing each other, so I don't need Stick to justify what he did.
Additionally, I understand Stick's POV on the Hand and so I understand why he felt he had to do it.
But that's not the way heroes operate. If they all did it Stick's way, Iron Fist would've been killed to keep him from the Hand. You can't have your heroes killing one another, even for the greater good.
Once the hero feels it's ok to kill for the greater good, it starts to get murky as to which criminals she/he can kill and why, and how sure about their guilt they need to be, and extenuating circumstances, etc.
Soon you get to the point where everyone's afraid the "hero" might come for them next. That's why they're not heroes. Heroes are cheered by the masses, not feared.