Though leadership varies wildly across companies, batallions, divisions, etc, but the most meaningful comes at the company levels and a piss poor leader can cause a lot of damage (mostly due to incompetence)
Our sister company was deployed to a different base w/ a somewhat different job -- they left before us (I spent my leave in the barracks w/ them but that is another story) so they got back before us. One of them raped somebody in the barracks and by the time we got back it was VERY different. MP regularily did patrols at night on the weekends, they did ID checks for those drinking, instituted a rule saying no alcoholic beverages outside of barracks rooms (the same barracks that had a 3 story beer bong before deployment) Some of it died down by a year but it was never the same as it was.
Your theory on psychopaths isn't a bad one, there were people I knew well that came in around the same time I did and scared me when they started moving up ranks as these are people that shouldn't be leaders. Luckily during my deployment my platoon sergeant was the best, smartest soldier I ever knew who had miles of integrity and was a much better, effective, and knowledgeable leader than 1SG(he actually accused someone w/ a leg injury of "malingering" who was driving a vehicle where the passenger side was hit w/ an EFP where the passenger died) or Company Commander. He didn't play favorites or had friends which was the most common problem w/ leadership(friends would get the least details, least trouble, and most opportunities to advance) He was also the Equal Opportunity rep of our unit and gave the greatest Equal Opportunity briefs of all-time(maybe an overstatement but can't see how you can do it much better than him).
On edit - I'd also add that CID is a very effective law enforcement division, their under-covers bring down a lot of people. MPs break the rules as much as anyone else so they can't really be counted on.
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