I have my own key to my neighbors apartment. He put a latch key lock on the outside of his apartment door so I could not go in and get my belongings. I took the outter lock off and opened the door with my key, grabbed only my belongings, and left. When he got home, he called the police and told them that I broke into his apartment and stole his camera. The cop came to my apartment and told me that I broke in and that she's putting in a report stating that I did steal his camera. Is it illegal to get my belongings if I have my own key?
It's trespass, and possibly burglary, and makes it easy to accuse you of stealing things. Give your neighbor back his keys and don't give him any more of your things. If the DA decides to press charges, get a lawyer to help you.
Criminal Defense Attorney
You have a right to your property but that doesn't mean you can burglarize another person's property to retrieve what is yours. The proper way to get this accomplished would have been to get the neighbor's permission, to call the local sheriff or constable when your neighbor is home to get their assistance, or to file a law suit to have your things returned.
Even if you have a key, that doesn't mean that you have a right to enter into another person's house or apartment. If they have told you expressly that you do not have permission (and/or have put another lock to prevent your entry), then you are going to struggle to prove that you had permission.
You are likely to have a burglary charge filed against you. I would strongly recommend that you stop talking to the police or your neighbor about this matter and contact a local criminal defense attorney as soon as possible.