Currently I find adventure mode to be a little bit broken.
That is the understatement of the year.
Although, I suppose "broken" really isn't the best term; "unfinished" is more correct. Eventually bandits will be properly hostile again, but right now they aren't as self-aware as they ought to be. The last update did a lot to make people's behavior less arbitrary - sieges and beast attacks actually need to set out and cross the map to reach your fortress, whereas before they were simply generated. People now need motivation to initiate an attack, for the most part, and apparently "being a bandit" was not included in these motivations.
Peoples reactions to you similarly need work: being a "killer" isn't actually a
bad thing, it just means you are recognized as someone who has killed, someone capable of violence. And if the townsfolk want to call you unhinged, just let them; it doesn't affect their behavior towards you.
Asking for beast-related trouble will eventually get you some animals, night creatures, and vampires, but you basically have to go through the entire list of megabeasts and titans before they'll tell you about any local troubles. I much preferred the old system, gamey though it was, where no one asked you to kill a megabeast until you had the requisite fame to speak to the lord.
If you want fighting, best place to go is probably a goblin pits or fortress, since goblins
will attack you on sight (mostly). And there's lots of them.
On the other hand, we did get non-human sites and new combat mechanics now, both of which are rad. I look forward to the next update, where it seems we'll have more to do in Adv. Mode than just killing everything - that should ease a lot of these hostility issues.