Originally Posted by Maylander
After experimenting a bit, I discovered something new - it seems the hostile trigger points are affected by your standing with the faction; the higher standing, the longer it takes for them to go hostile after you start killing off cities. The numbers I posted above are at roughly 0 reputation with the faction you are attacking - at 30+ reputation it takes a lot for them to get hostile, I cleared 5 Hashishin cities at 34 standing and in the 6th city they were still friendly.
Quite interesting really. This means that getting a little reputation with all factions before you support someone is easier than just attacking right away.