In the first act when you click on a rat in combat mode, it should pop up a message asking if you wish to turn hostile against all rats, and you can either continue your attack or cancel it. Since this seems to be causing a problem, have you tried doing a clean install of the game?
Backup your saved game folders, just in case, then uninstall, reboot, make sure there is nothing in the
Beyond Divinity folder outside of the
savegames folder, then shut down all non-essential programs (especially anti-virus), re-install the game, reboot, shut down all non-essential programs, install the
patch and try playing.
Maybe start a new game, cancel out of the initial conversation and see if you can attack a rat.
After killing Fergus, try letting the imp go. It might not have any influence on the rat problem, but you get more experience doing it like that anyway.
