I think starting a thread as opposed to bringing one back from 2012 would be a start..
But to answer your question. By not securing it to something your making it easier for someone who broke into your apartment to see a shiny big box of guns which they can remove and worry about breaking into at a safer location. using proper hardware there won't be much damage to the wall, nothing a little hole filler can't fix when you move. Other option would be to use a cable to go through the bolt holes which can be lock around something that can't move, at least while attached to your safe. I would go with securing the cabinet properly. As far as the rules with landlords and all that Jazz I can't help, but again, starting a new thread directed at your issues might get you more help..
Cheers
Welcome to the site..