It would, at least a bit. But I'd have to consider it alongside the other stuff I knew about the candidate. If some candidate had attained the office of, let's say, the governor of a large state while reliably carrying out my policy preferences and not getting embroiled in a major scandal of some kind, I doubt I'd give much credence to the hypothesis that they'd controlled their sociopathic urges, biding their time until they were elected US President and then unleashed terror upon the populace. Also note that there are enough veto points within the structure of the US government to prevent any one person from carrying out Stalin-level genocide or whatever; your point would maybe stand a bit better if we were electing a dictator, but then, dictators don't get elected.
dictators don't get elected
Sometimes they do!
I wrote an article for h+ predicting that the rapid fall in the cost of gene sequencing will allow U.S. voters to learn much about presidential candidates' DNA. The candidates won't be able to stop this because:
DNA analysis has a decent chance of reducing political bias by providing objective information about candidates. If, for example, 70% of the variation in human intelligence is determined by identified genes then DNA analysis would reduce disagreements among informed voters over a candidate's intelligence.