Maxperson
Morkus from Orkus
So you can notice a slight change in wording, that slowly creeps into this spell with each new edition. At first it was instant death, and with 3rd edition it actually became damage based. The Forgotten Realms wiki states it as follows:
"Older versions of Disintegrate made the target disappear no matter if it was a creature, or magical matter or 1" cubic volume of other material. The newer versions spell struck and injured a target, and if killed, the ray caused the creature to disintegrate into a pile of fine dust, although any equipment is not affected. The ray can also disintegrates as much as one 10-foot cube of nonliving matter, even objects constructed entirely of force, but cannot effect magical effects such as a globe of invulnerability. Only one creature or object can be affected per casting."
No, this is incorrect. It IS a pure damage spell from 3rd edition and upwards. The Forgotten Realms wiki backs this up. Starting with 3rd edition, the spell only does its effect at 0 HP or less. This is because in 3rd edition you don't actually die until you are at less than -10 hitpoints. So any damage that would put you in the "dying" or "bleeding out" state, triggers the effect. But it does require the damage to bring you down to the level where you would be dying.
It has no longer been the insta-kill spell for two editions already.
3e says 0 or fewer hit points = disintegrated. 0 doesn't kill a PC, but it does disintegrate him. 0 is also not dying in 3e, so that is wrong as well. You have to be -1 or greater to be dying. So no, it's not pure damage, nor does it need to kill. The FR wiki doesn't change that. At best, it becomes a house rule for the spell for that setting only.