So I had recently fought through the Nosferatu tunnels and gotten the artifact the local prince wanted.
When I made it back to town, though, it appeared that either my party members were going hostile on the populace for no reason, or the populace was going hostile on my party members. Either way, quite a lot of people died. I'm not sure why this happened, since I'd been able to walk through town unmolested before with my party.
Any suggestions on why this is happening? My own searches haven't revealed much, and I've never played this game in the past.

Haven't played this in ages but double check that none of your party are frenzied, have feral claws or beast form on or anything like that. Frenzied can be caused by enemy powers and wilhelm can use feral claws, cant remember if any characters have beast form by that point.
Not sure these powers or states carry over area transitions but you might want to check before you head into a populated area.
Now if only a Customer service rep could sort out my installation problems I could play this again

Hmm, they might have been using feral claws...can't recall. I also wonder if my new suit of unholy armor was the cause...I'll have to experiment...

It's quite possible you turned your allies to aggressive.
The options are on the left side of the panel. One is a shield then a half filled in circle and then a sword. If that panel is set to the sword option your allies go psycho killing everything in sight. Switch it back to either of the others when in town and your should be fine.

You likely have your party mode set to aggressive. It needs to be at the lowest setting (green dots as I recall) when in town or your party will attack everybody drawing the guards.
Hmmm, not sure how I missed "komisches"'s answer - looks like you already have it (which is good :) ).

If you play the game in neutral, your coterie won't attack in the cities (as long as you don't feed).

In neutral, your characters will attack the one you're trying to feed on (in the city).So if you don't want them to attack as you feed, set them to defensive.

I couldn't ever manage to feed in the cities without being attacked (even if nobody else was in sight). I just stocked up on blood and fed that way in the cities. You can feed on the monks in the monastary even when they are in sight of eachother, but check their blood pools. They didn't always seem to regenerate when I returned for more.