I interpreted his remorse over the decision as genuine. He was conflicted about it (doesn't excuse what he did of course) and when he realized Clarke's mind is alive, he wanted to reverse it. Not that I understand his decision to kill Clarke but I can accept his POV, he might have been afraid he wouldn't see his daughter again for a very long time, the dire lack of nightbloods and whatnot. Maybe he had also forgotten what a scoiopath she was and now is kinda like huh, maybe it would be a good thing to put her back on ice LOL jk, he thinks with artificial nightbloods, he won't have to worry about this problem ever again.