There was no way Clarke could kill Lexa, all the grounders were around, she would be killed before she pulled the knife. The people from the Ark have guns, probably a decent amount of ammo, and at least some of their people know how to use the guns. But when say 75 people with automatic weapons and a few thousand rounds of ammunition are facing what could be many thousands of "primitives" who have demonstrated considerable capability with very lethal thrown weapons. Someone will either run out of ammo or get overwhelmed/killed while reloading, which takes a gun off the board, weakens defense in that direction, etc. We've already seen that the Grounders are willing to die in battle. This is when the show gets serious, now the story is really rolling.
Review of Spacewalker
