You can't vote against a candidate. The ballot doesn't work that way. You can only vote in the affirmative.
As far as Nader most people make the mistake of assuming that those people would have shown up to vote for Gore had Nader not run. I doubt that. Nader inspired people to get out and vote for him despite the inevitable loss. You think they would have bothered to go vote for a guy when they picked someone destined to lose over him? They would have abstained like the majority of Americans did. You can't blame Gore losing the election on Nader. You can probably blame it on Jeb and the SCOTUS but not on Nader.
Had they known how few of those Nader votes would have handed Gore the election I guarantee many Nader voters would have voted Gore.
Are you saying that a vote for Hillary isn't a vote against Trump?