You're quite right. It is a repeating feature of the bigger extremist/revolutionary groups that they share a lot in common with mob/Mafia-style organised crime. Simplistic or idealistic messages of a better life or protection for the downtrodden, a certain popularity in the early stages (blind eyes turned to aggression), and then the inevitable dissent which is brutally suppressed with violence and terrorism. George Orwell's 'Animal Farm' was inspired (if I remember rightly) by the Russian Revolution which started with high aims and 'acceptable' violence but ended up with mass slaughter and imprisonment for no reason except to create fear and maintain control. You can trace the same behaviour pattern through Stalin's Russia, Nazi Germany, the IRA, Mugabe's Zimbabwe,
The people taking the moral high ground often end up hostage to their foot-soldiers. If anyone suggests pulling back from the hard-line tactics, they are at risk of becoming a victim themselves. So it self-perpetuates. In the case of the Taliban - or more broadly the whole 'islamist' death cult phenomenon - where there is no clear leadership, hierarchy or aims, conditions are perfect for small breakaway groups and angry individuals with a blood-lust to simply make it up as they go along. That's how you get people shot dead in Australian cafes or blown up running the Boston Marathon.