You can wonder, but what if you are wrong?

Even if the this was a simple case of a man and a woman arguing, the man attempting to intervene was trying to do the right thing. And he was stabbed multiple times and left to bleed to death. Even if the man was butting into a situation where he had no business, should his punishment be a brutal execution?

Gandalf said:
Many that live deserve death. Some that die deserve life. Can you give it to them, Frodo? Do not be too eager to deal out death in judgment. Even the very wise cannot see all ends.