It was quite clearly suicide because the underlining reason for killing themselves was pure selfishness, as suicide is the most selfish act a person can do.
As with the kamikazes in WWII, the suicide-pilots were taught that by killing themselves for their faith would bring them into a certain type of "heaven;" besides the fact that they knew they were fighting a war they could never win and in desperation (a prerequisite for suicide) killed themselves to seek "eternal notoriety," another selfish act.
HorseHillary! The underlying (I'm assuming this is what you meant) reason they flew into the buildings is because there was no other way for them to accomplish their missions than to sit behind the controls and pilot the planes into them.
Or do you know of some way for them to have accomplished this and survived?