Bertolt finds himself nodding. When they had found Marco and Jean, his heart went out to them. It was obvious that they had a hard time, but honestly who hasn't? They've both lost people they cared for, and in Marco's case...

A weaker man would've given up by now.

"We all were at one point, Reiner." Even with their skills, they had to adapt too. Following an order to kill someone on the battlefield was one thing, but killing anyone who was a threat (walker or human) was just as, if not harder. Friends turning on friends. Humans turning on each other. Friends turning into walkers. "I just... I can't lose you. If they find out and they try to hurt you, I..." He wouldn't hesitate like he did with Berwick.