Economics

Yes 'humans' do this but I still think it doesn't have to be this way and a majority is not bad.
It is unfortunately easier to do something bad than good.
It's the sociopaths/psychopaths or simply greedy aggressive people who don't think twice about being ruthless and not playing by the rules that acquire power. The rest simply is put in a position to follow orders.
None of the people fighting wars want this, the people far away from the front who send them there do.