While not the worst gun ever, the worst gun I've ever bought is a P82 Wanad, which comes from 1980s totalitarian communist Poland and is the kind of sidearm those goony gas-mask troopers from City-17 would carry while brutalizing people. I've had problems with the safety getting stuck, it's heavy, the ergonomics are uncomfortable, and it has a really heavy and creepy trigger pull. It was really cheap though and otherwise works. Could probably really bust someone's head open by bludgeoning him with it, which was probably the intended purpose.
https://youtu.be/8awXGkgW1vI