From The Atlantic online forums:
Sure, he killed a pimp and some other unsavory characters and "rescued" a teenage prostitute. However, how does THAT absolve him of attempting to murder a presidential candidate?
The guy reaches for something in a coat and the security men run after him; they don't have a name or a picture or anything, just an eyeball ID. I doubt if they would connect it with the later shootout, much less try bring charges.
As for what's the point of the ending, you got to go back to Dostoevsky, from which it was in part inspired. His characters, much like Bickle, are loners so out of touch with society they need to do something to let themselves in, or at least, let themselves be noticed, like shooting a presidential candidate (I also think he believed the candidate was somehow responsible for making the love of his life--Cybill Shepherd's character--reject him).
The final violence is supposed to be the consequence of all that obsession--Bickle's frustration and loneliness finally coming out in a blaze of bullets and fountaining blood, so to speak. He wants this so much for so long, and finally he gets it.
The finale is the ironic point of the movie: Bickle the time bomb exploded and the result is that he's a hero. Now that's really scary, because as crazy as Bickle is, and we've spent enough time inside his head to know that, the ending shows us that the city is crazier.