Hussar
Legend
Hostile in the sense that they will actively work against you. IOW, the roadblocks that DM's continually place in the way of the players in the name of "believability" simply reinforce the very behaviour that these same DM's complain about.An NPC is "hostile" if they attack you or clearly intend to do so. If a PC decides to strike preemptively, that has more to do with them than the NPC.
Not that there aren't plenty of reasons why that might happen.
If every NPC will automatically treat the PC's as untrustworthy and "be leery" of them, resulting in the players constantly being frustrated by trying to resolve situations in anything other than just killing everything, then the players will default to the path of least resistance.
Like I said earlier in my response to @Lanefan - if the fleeing baddie will always result in a negative, then the players will never let anything go. Why bother talking to anything, taking prisoners or letting things run away if it just bites them in the ass?
If you want players to actually behave in a particular way, then that particular way has to be rewarded in some fashion. If it is only ever penalized, then no rational player will ever do it.