The story seems to be that he had had a few run in with the Angels earlier in the day, probably because he was with a white girl. It's important to put it in the context of the time i.e. racial tensions. So he was already fired up so to speak, obviously the Angels by the time the stones hit the stage were well intoxicated and tripping their tits off, so it's a theory that he pulled the gun at one of the angels rather than Jagger. From the film it does appear that he is pointing it toward the stage, but who is to say he wasn't taking aim at an Angel on the stage wings, maybe one that had done something to him earlier oin the day. If you think about it, if you were going to shoot Jagger, would you pick a gig like Altamont to do it? Surrounded by Hells Angels? No, you would do what the guy in the film does when Jagger is arriving and he runs up and hits him. More likely you would get an accurate shot off than in a crowd which was moving around and at a target which was also moving around.
For me, the whole thing is a result of, Hells Angels who were not in a good mood and off their heads, racial tensions carried over to an individual and a bad vibe in the evening generally. I once spoke to a guy who had been at Altamont and, whilst he wasnt anywhere near the front, he said the atmosphere was awful and got worse as the day went on.

'some things, well, I can't refuse'