I don't blame Nader voters, because they didn't realize what could happen. Voters in 2016 have no such excuse.
The main argument for voting for a guaranteed-to-lose third-party candidate is that it gives the parties a clear, quantitative signal about how many voters their base-unfriendly policies are losing them.
And also the down-ticket races, of course, though many of them are so gerrymandered here that your votes don't matter in them, either.
This isn't a counter-argument. Yes, there are innumerable ways Gore could have gotten the handful of votes needed to push him over the top in Florida.
But it's difficult to argue that, if Nader voters voted for their second choice instead, Gore wouldn't have gotten enough votes to win. To argue otherwise is to argue that a majority of Nader voters would have picked Bush as a second pick, and that's difficult to imagine.
Voters in 2016 are the same. Not liking a candidate doesn't mean I should feel any obligation to vote for their "main opponent". I'm voting for who I feel represents myself and my beliefs best, and if the candidate I dislike ends up winning, that's on the ones who voted for him.
My vote is the voice I get in democracy. I'm not going to use it to vote in someone I don't agree with on most issues, just because the other guy is worse. Me withholding that voice (or using it to vote for someone I know won't win, but who i genuinely believe in) is the way that I can be heard. It's my way of telling future politicians that if they support X (or something close to X) that they can get a portion of those who voted for C last election.
It's my way of sending a message that the next candidate shouldn't be a serial killer AND shouldn't be involved in tax fraud. It's my call for someone who better represents me and my beliefs.