While Thorfinn is right regarding peace, you cannot just shrug off what has already occurred or your people's feelings, you yourself brought over there promising them a better life.
Thorfinn talking about leaving so casually, makes it sound easy like he is a lone traveler or something, while he brought so many people with him. Who is going to keep following someone who despite having good intentions puts always his own ideals above his own people's needs.
Einar's expression says it all. Dude changed after the first kill. A friendship might be ruined for good there.