Well, if I HAD to vote, I'd pick the least obnoxious candidate, but we wouldn't really be living in as free a society as we do then. But to address the spirit of your question, let's say a bunch of people don't vote, because they don't support any of the candidates. So only those who do, end up voting, and things get worse. Let's say they get way worse. I don't know about you, but I've noticed that individuals--and thus societies--don't tend to address a problem until they have to. People have to hit "rock bottom" a lot of the time before they address the issues and move on. Sometimes societies do too.
When an individual's behavior is clearly going to lead to a big fall, and you are the only one stopping it by constantly picking up their inordinate amount of slack, by removing the obstacles, putting out the fires, and shouldering the weight of their problems because "who else will?" we call that "being an enabler." There are societal enablers just as there are personal ones. Yeah, it affects people when someone hits rock bottom, and it affects far more people when a society does it, but some means of "preventing" this only serve to mask the problem and delay it rather than actually prevent it.
There is always the third option of saying "these options are not acceptable." That's how civil rights worked--if someone said, "All the black people have to sit at the back of the bus or the right side of the bus, and those are your choices," you would not sit and debate which is the lesser of two evils and then pick one. You would declare both options unacceptable and not participate except in a way that makes this clear, and work to raise this awareness.