You're making that choice right now. If you stand on a hill with a megaphone shouting "GENOCIDE JOE GENOCIDE JOE!" for 8 months and then at the 11th hour decide to go "eh okay I'll vote for him", it's too late.
if you're worried about splitting the vote, and you know people won't vote for joe because of his genocide, it's incumbent upon you to either get him to stop the genocide or support some other candidate.