Not yet, but considering the sheer amount of guns in his hotel and home plus the explosives I'm willing to bet something motivated him more than mental illness.
He's not just a random crazy person that bought a gun and went on a spree, there was a lot of planning involved, so he's probably a terrorist of some sort.
Deliberately planning mass murder doesn't make you a terrorist, just a mass murderer; terrorism is defined by a very specific kind of motivation that is far from the only motivation for planned mass murder.