In a situation where there are enemies who want to conquer you all around, the best person to be king is the best leader of an army. That is why it made sense for Robert to be king. The problem is when Robert dies then the only way to find out if his succesor is the best at leading an army is to have a civil war and let the winner be king. That is untenable so hereditary monarchs make some sense, because it avoids a civil war and hopefully leadership ability may be passed on from the father.
The best situation is a republican form of government, but that requires the military submitting to civilian authority which was relatively rare in history until a few centuries ago.
“Democracy is the worst form of government, except for all the others that have been tried from time to time.” - Winston Churchill
Hodor Mother’s Day, everybody. 
Plus don’t forget he’s the product of incest so he may have a touch of madness from it.
I can’t believe that one generation of incest could be blamed for psychopathy.
Well Tywin also married his first cousin, so Joffrey has a lot less ancestors than most of us.
Interestingly (if you’re a nerd like me), while we often think of monarchs being descended from a long line of first cousin marriages, that didn’t become established habit in Western Europe until after the Middle Ages. Western European Medieval society considered first cousin marriages incestuous.