"Si vis pacem, para bellum."
"If you want peace, prepare for war."
It sounds like an oxymoron for pacifist and anti-war supporters. But it reflects reality and is necessary for survival.
One cannot be truly peaceful without being capable of great violence. If you are incapable of violence, you are not peaceful - you are harmless.
The irony of Thorfinn's pacifist beliefs is that he will survive hostile encounters because he has lived through war; it's his people that will suffer and die.