It could be one of four things really.
1) Terrorism - Not particularly likely as these sort of people tend to leave statements to be picked up after the incident, be it on a website or through mail or some such.
2) Murder/Suicide - The most likely, sadly, and it's not easy to find the evidence and reasoning for it. His life and communications are going to be poured over with a fine tooth comb, his family are going to face weeks, if not months of questioning from both authorities and the media.
3) Health issue - The second most likely, although it does not explain the locking himself into the cockpit. In regards to what kind of health issue would cause such a thing, I refer again to the Moorgate crash.
4) A combination of number 3 and an accidental locking of the door - Very unlikely, I don't know how cockpit doors work, whether it's possible to lock yourself out of the cockpit, I doubt it but if the pilot accidentally locked himself out and the co-pilot had a health problem then that could feasibly lead to a crash. However, it would have to be a combination of the two, and very unlikely.
Sadly, it does look very much like a murder/suicide, and there are no words that won't result in an infraction to describe the type of...thing...that would kill 149 innocent people just to end its own life.
