I understand how upset you must be...with that said, you said you thought it got in through the top corner of your gate....a-typical for a raccoon! Is your gate chain link fence also? Can you post a pic of your area so I can get a better idea to help with beefing up your setup?
Most likely it was a raccoon or fox. Can you post a pic of your coop and run so we can see what's up? Also, what kind of wire are you using on your coop/run??? Do you see any footprints?? Do you see where/how they got in???