A whole, whole, whole lot of people have held political power other than those guys, and with very few exceptions have restrained themselves from committing mass murder.
But the expect cost of a sociopath becoming the U.S. president is huge. Also, although I don't agree with this, lots of people think that George Bush was a mass murderer.
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.