First off, he (and everyone else) has the right to run. He has the right to give you another option, in a system where most of us feel two options aren't enough. (We make fun of countries with only one option, but there's not much difference, really.) He has the right to run, and you have the right to vote for him, or not.
Our system doesn't recognize "would have been" votes. Either you vote for Bush, or you vote for Kerry, or you vote for Nader, or you vote for someone else ... but nobody cares what you would have done if options hadn't been available to you. Nobody except those who like to complain. Sure, we could have a system in which you selected your top choices, and your vote could be automatically converted when your favorite candidate got disqualified -- but that's not the system we have.
If you want to blame people for votes for Nader rather than your preferred candidate, remember to blame the people who vote for him rather than your candidate. And if you prefer Kerry, remember to also blame everyone voting for Bush, because they didn't vote for Kerry. Or would you rather blame Bush for running? (Oh, wait, you probably would. Heh. Sorry.)
Our system stinks. But until we can fix it, at least remember to play by its rules to get what you want, and don't blame others for doing the same. Some people really want Nader. That's who they vote for. They like him enough to vote for him despite the chance they'll wind up with another candidate in office that isn't their preferred alternative.
Sure, our polls show that Nader doesn't have a chance -- but we don't actually know that until everyone's voted. You can't disqualify a candidate before the vote because you're just sure they can't get enough votes to matter. (Well, we sorta do that: you can't run without a certain minimum number of signatures -- which reminds me, we really should all be willing to sign to let anyone run who asks, to have more options. You can always tell them to go screw themselves once you get to the voting booth, but hey, variety's good.)
When the vote doesn't go the way you like, you have two things to blame: the system that counted the votes, and the people who cast them. Don't blame the candidates.