[He has many feelings about how that whole mess went but he will not burden sweet newly-dead Mathis with them, so he just reaches out to rest a hand on his shoulder.]

You oughtn't hold the full weight of responsibility. The events that unfolded weren't only due to you. Regretting certain actions or inactions may be something that you'll have to carry, and I know this is a difficult ask, for now, but... try to keep it in mind. Even if things didn't go perfectly, you still tried quite hard.