There are some survivors that will attack you. They're the ones wearing orange prison jumpsuits and you get a bounty for killing them. That's not a bug, that's part of the game.
I use a fire tower near the entrance to deal with Ninjas. It makes them use their invisibility early, and as soon as one goes invisible it will switch to the next one, without having to wait for the reload time the other towers have.
 "Zombies Took My Daughter!"){kind=icon}
![Play [Visible] III](https://cdn4.kongcdn.com/game_icons/0018/1532/Thumbnail.png?i10c=img.resize(width:250) "[Visible] III"){kind=icon}
 "The Red Baron 1918"){kind=icon}
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 "Protector IV"){kind=icon}
 "Heidi Montag Goodies Underneath It All"){kind=icon}
 "Cursed Treasure: Don't Touch My Gems!"){kind=icon}
 "Toss the Turtle"){kind=icon}