I wasn't of age to vote yet, but I probably would've when I did because:
He didn't seem as bad yet. The Russia-gate case didn't really have a lot of oublic evidence yet, and you could brush off some of the dumb things he said as just being for the campaign.
HRC looked just as shady as him, if not more. In hindsight we know that he's a bit more shady than her, but as I said, we didn't know that yet.
The anti-trump crowd was (and still is) insufferable. Sure, there are a lot of retarded trump supporters, but they don't get the massive media platforms that the anti-trumpers do. Seriously, they latch onto everything they can to try to "BTFO" him and his voters, and then put it onto any form of media they can. There are subreddits in the double-digits that are devoted to constantly x-posting everything trump or his voters do.
Also, pro-tip for this crowd: Insulting people makes them want to agree with you less, not more. People won't concede a point if you attach a "you're a bad/stupid person" to it.