Elections are pragmatic. Who is most likely to enact policies that you agree with? Note - this question is different from asking who you agree with the most or who you like on a personal level. Winning is a prerequisite - if they have no realistic chance of winning office, then they cannot enact any policies.
So the question is simple - do you agree more with the vision for the US laid out by Trump or Clinton? If the latter, then pulling the lever for Hillary is a no-brainer. Who is going to fill Supreme Court vacancies with progressive voices? Who one is going to protect the EPA's mission, rather than gutting it? Who is going to protect gay rights, women's rights, and reduce income inequality?
Hillary is a deeply flawed candidate, but like her or not, at this point, she's the only viable candidate who can keep this country moving in a progressive direction.