Do seat belts kill?

If worn incorrectly (tucked under armpit for example) and a collision happens then yes you can die even wearing a seatbelt. The belt can press into a person's stomach hard enough to do damage (compressed lungs, burst intestines, pressure damage in general).

Even worn properly people can regularly get cracked ribs in car accidents, however this is significantly less damage than would be incurred if you were thrown out of the car window as a result of not wearing a seatbelt in a collision.

Seat belts can possibly decapitate you. As such can occur if a crash is violent enough. Luckily, most are not.