I dont think it would be evil ,, in some areas it is legal to hunt ferrel cats and in some countries thay eat cats , you might want to look into some of the bolt tips made for small game though, Montec makes one not sure of the exact name of it,, I would say the cats are just hungry,, maybe they think you are foodor they have dubbed you as a kitty treat,, or tidbit.. if they are attacking you I would dispose of them also I might would do it a different though ,,
myself I would go to the local humane society and ask them for help they might give you some box traps to catch them in,,

I do thik you might catch some heat from some.. just if you do decide to take them out with your crossbow please make sure of your shot so there is not an injured animal suffering..