But also remember too, they are good for fighting among themselves.
They don't do it for entertainment. For instance, Hamas doesn't fight with the PLO for fun - it does so because the PLO stands in the way of Hamas's domination of the Palestinian population. (We are sponsoring the PLO because the alternative is Hamas). Similarly, Sunnis in Syria are hoping to stamp out the Alawites, much as French Catholic nobles exterminated the Cathars during the 13th century Albigensian Crusade.
These things happen for the same reason other wars happen - the parties disagree on who should rule. Stalemate occurs only when the parties are unevenly matched. In Afghanistan, if the Taliban hadn't helped al Qaeda stage 9/11, it would be the ruler of all of Afghanistan today, having finally overrun the Northern Alliance after over a decade of significant financial assistance from both Pakistan and wealthy Gulf Arab donors (possibly hoping to develop private armies they can use to seize power from the existing Arab rulers).