Good Evening momen37 and welcome to Apple Support Communities - I'm sorry to read about the issues you are facing. Whenever you purchase a second-hand iPhone, it is important to check for Activation Lock.
To remove this you will need Proof of Purchase, if you have bought it from a person directly, you will likely not have sufficient proof to satisfy Apple's requirements (This whole policy is in place to prevent stolen phones, which in principal is a great idea but sadly it seems to effect people in situations like yourself)
How to remove Activation Lock – Apple Support
Are you able to contact the previous owner again and ask them to remove their Apple ID? I understand he has previously been unwilling to help you, but if you try to explain your issues with the device then hopefully he/she will be kind enough to help you.