Monster infighting only affects Cupids and Fallen Cupids/Imps for now, since they are more directly opposed. Other forms fighting each other may or may not be implemented later.
We had a look, and it turns out if you're playing on the lowest possible grapahics settings the checkmarks are actually invisible. Is that what's happening to you? Can you try playing with the graphics on low instead of "as low as possible"? Even if you're not on the lowest one, just try different settings for quality (and maybe size too?) and see if you can make them show up. If you can make it work, please let us know your settings on which it DIDN'T work so it can be fixed in the future