Vulnerability gets you killed. Vulnerability makes weakness, and weaknesses are exploited and cause you to be beaten down. He couldn't be vulnerable, that would lead to certain death.

He couldn't be the kid who was vulnerable, who was scared. He couldn't be the teen who'd flash a brief moment of being brokenly vulnerable.

A mask, a killer with a mask showing no weakness. Nothing for others to prey at, nothing for someone to attack. He got so used to that, he forgot how to open up. He forgot how to even BE vulnerable.

Not even Severin has seen his brother at a fully unmasked state.