Hi Luca, I suggest a trainer, experienced with german shepherds, working individually with you and your dog on these problems. I'm wondering if it was a territorial issue when your dog discovered people in his park. If you aren't feeling confident walking your dog at night now after the incident, your dog will have picked up on this, causing him to be protective/aggressive. You need to have an attitude of confidence, leadership and control. Regarding the cat. I'd separate their living areas when you aren't there to supervise, and always have your dog on a leash and correct him when he thinks about starting on the cat. He needs to learn 'Leave it' regarding the cat. It's your home, you are the leader, and going for the cat will not be tolerated. Regarding an ecollar, do your research and learn how to use it correctly. If your timing is off, you can make the problem worse. The cost for a good one is $300 Australian.