It's a knowledge as devastating as the mechanical click of a trigger, and doesn't he know all about that? Erik does. Charles blinks his tears away and rallies himself around the ballistic damage with a shudder. Erik needs strength, because weak honesty is honesty without conviction, and that wouldn't be nearly enough.
"You're capable of great things, Erik," Charles says, firmly and quietly, holding Erik's gaze as closely as he's allowed to, "You are a good man. You have the potential to change the world forever. And yes. You're capable of violence, but that you don't need me to tell you. What you aren't capable of is atrocity. It isn't in you, Erik, and I'd know. Of anyone, I'd know."
He strokes Erik's temples gently, careful not to disturb the slicked armor of his hair. "Everyone is capable of evil, Erik. It's in our nature, but it doesn't make it our fate, and I know you have the capacity to be cruel. Just like I do. That doesn't make you a monster. Nothing could make you into a monster, love, not after all you've already been through. That's not something I could say."