*Spoiler* Here's my theory. Yes, No, Maybe?

This is what I think happened in the film. I'd like to hear other opinions.

Because of the horror of his experiences, Saul has completely disassociated himself from his past. It's unimaginable to think of what these people had to do to survive. I think he snapped. I believe that's why no one wanted him to be a part of the escape plan. He was too unpredictable. Something changed in him when he saw the young man killed. By giving his "son" a proper burial, he could recapture his own humanity. It would be one last chance to right the wrongs and quiet his guilt. I also believe that Ella was his wife, but his disassociation wouldn't allow him to recognize her. My theory is that's why she reached out to touch him, but he refused. Had he allowed that reconnection, he would have to relive all the horror and his choices again.

But then again, who knows? That's what I love about films like this!