An autonomy/consent perspective strongly distinguishes this scenario from assisted suicide. You do not generally forfeit your right to bodily autonomy just by doing something that's dangerous or that you mean to die from, and shooting the falling man might interfere with some way in which he was exercising autonomy-- you might keep him from being remembered as someone who wholly chose the moment of his death, or you might deprive him of a few seconds of weightlessness that he had intended to be his last experience.
Of course, you also might NOT be interfering with that, but you don't know, whereas an accomplice (partner?) in assisted suicide is arguably acting to support the dying person's autonomy. (One good elaboration of this argument is Ronald Dworkin's "Life's Dominion".)
I'll mention two ways in which I think you do forfeit some autonomy by attempting suicide, though I'm not sure either one matters here. One, you might be reducing the weight of your interests in any kind of balancing that has to be done; if your suicide is going to break someone else's leg (which jumping off a building might!) then atomizing you with a ray gun on the way down is probably more than justified even if it thwarts an exercise of your autonomy. Two, if your suicide looks like an accident, good samaritans will probably feel free to do all kinds of bodily invasive things to save you, if they think you can be saved, and I think that this is reasonable by most people's lights.
(There are plenty of philosophers who would say that preventing a suicide is an act of good samaritanship even if you know the person wanted to die. But plenty of others would disagree, and it depends among other things on whether life is an intrinsic good, which is a whole other kettle of fish.)