It was my grandfather. The man who I respected so much, who taught me everything I know about medicine... grew old, and his mind began to deteriorate. I could not stand to see him as he was now, and so I killed him.

Which - would be awful and contemptable enough, on its own. But if we don't find some way to change Aliza's hibernation cycle...

[Then he'll be in that same position again, and he doesn't trust himself.]