There's an event going on right now on Siege (possibly others, but who knows/cares) where Rabid creatures/humanoids are spawning in certain towns.  These are red, powerful creatures with many thousands of HP, capable of inflicting lethal poison.  They are immune to guards, but passive to NPCs.  When killed, regardless of what creature they appeared to be, they leave wolf/dread spider corpses with appropriate (poor) loot.

If you don't mind taking the guardwhack yourself, you can discord them to get their barding difficulty below 160 then provoke them onto townspeople.

Are there any townspeople worth killing these days?  Any rares/semirares to be had, even if they're rather useless?

Also, I guess you could use this to kill people in towns, but anybody AFK near one of these creatures would eventually be randomly targeted anyway.  Lure one all the way to another town, where people expect to be safe? I'm sure there are shenanigans to be had.

I can't think of anything worth killing that isn't marked invulnerable.

They're also immune to taming and Dryad's Allure.