"The thorns react only to attack." He lifts his eyes to her. "And I have not decided whether to be their enemy." And this statement is deeply telling, revealing much about his state of mind: he has not decided, because part of him truly wants to be the hero. But next to Sif, next to someone who knows him and who expects the worst of him (because, don't they all?) means that being a hero is impossible. And the call of chaos is keen, and sweet, and it would do him well to be a villain.