He can be both.
No, he can't. When someone is "nuts", he's not capable of making rational decisions. That's what it means.
I see no evidence at all that he was "nuts", unless you argue that somehow every mass murderer is "nuts". All evidence we have seen so far describes a wealthy, retired accountant -- a tough profession if you're "nuts" -- and successful poker player -- again, very difficult for a "nut" -- who made the calculated choice to murder a lot of people.
That's called evil.