http://www.ted.com/talks/steven_pinker_on_the_myth_of_violen...
Seems pretty persuasive.
To think that humans do these things out of hate and dehumanization puts the cart before the horse. Rather, there are cultural drives that incentivize certain behaviors, leading people then suppress and rationalize away their empathy in order to engage in those behaviors and reap the direct or indirect social benefits.
The most common pattern is devastatingly simple: hating an out-group is a powerful way to create an in-group bond.
For other morbid Medieval executions, read about being broken on the wheel. It's hideous.
I'd love to have a link to that talk, by the way.
http://www.ted.com/talks/steven_pinker_on_the_myth_of_violen...