Not really. Labeling him as mentally unstable is a perfectly valid assumption. Trying to say something about exactly what was going through his head is something I won't do and nobody should do, because quite simply we have no idea. Most likely none of us have ever been in a situation where those kind of things have entered our mind.

But mentally unstable? Yes.

Something like this that happened, is an exclusively human event. You could take the most ruthless and efficient killers of the animal kingdom, and if you were able to explain to them what this person did, they wouldn't make sense of it. Killing out of necessity, out of protection, for survival, etc. is one thing. Something like this simply cannot be done by someone in a good state of mind.