I think that's an oversight, but if someone tried to kill me I would defend myself.

Also when you get inside the boss room, it seems unfinished because you can see all the outside stuff of the game and you can even walk off the boss area, though the ground changes colour, so I guess in a newer update there will be a kill wall or something like that to prevent the player from wandering off.

I would actually be very interested in a Mrs. Slithers boss battle. Could be fun if executed correctly.