This one is pretty simple. Say you’re a standard-bearing GOP voter who is absolutely disgusted with the candidate that your fucked-up primary/caucus season selected. Your last name doesn’t have to be Bush here. You could be my dad or my brother, but you’re definitely #NeverTrump.
Trump has the temperament to be a lot of things, but I think that the most likely one is “Andrew Jackson re-incarnate”. Jackson was so terrible that his party threw him out, leaving him the only President without a political party. The GOP should recognize that and bar him from office in the first place.
The electoral math is simple: if you vote Republican, you can choose to not vote or vote third party, at which point you make Trump -1. But if you choose to vote for Hillary Clinton, you’re making it -2, as she gains while he loses.
I support Sec. Clinton for office because I have become far more liberal over the last decade than I ever expected. I would encourage you to make your vote count double in November if you’re the sort that simply cannot stand Trump and want to push him under whatever electoral rocks he slithered out from underneath. That said, I respect your right to just go -1.
[An aside: I would expect Clinton to be a one-term President unless the GOP implodes. She’s pretty polarizing and no spring chicken. I want to see what she can do, though, given the chance. 18-yo me is so, so confused.]