I've heard a lot about the supposed evils about not voting/voting for a 3rd party in recent political discussion here in the States. The reasoning seems to posit that (e.g.) in the Trump/Hillary race, refusing to vote for either party seems to morally support Trump, and voting for a party who is predictably never going to win is seen as just as of an immoral approach (within the context that a community of voters don't want Trump in power). Essentially, although both of these solutions would probably ultimately give Hillary a greater advantage, some voters would declare a failure to actively condemn/act against Trump immoral.
I've heard similar ethical reasoning in social situations. For instance, if a child is being bullied at school, and a bystander does nothing to help him, some might say that the bystander was acting immorally not because an indifference to helping people is bad, but because refusing to do so condones the bully's actions. Take another example from biblical hermeneutical disputes. Some say that the last passages of Colossians 3 condone slavery by remaining silent on the justice of the act.
Here's my question: Where is the line drawn between immoral inactivity and a simple lack of action? It seems to me that if I were to carry such principles to certain extremes, half of my life would all of a sudden become immoral because I would be somehow indirectly supporting some all-permeating kyriarchy, for instance, by not acting explicitly against it.
Additionally, Who are some thinkers who have written on the subject that I might be able to read?