Is weight lifting bad on the knees?

Depends: It depends on how much weight you are lifting and what kind of lifts you are doing. If you are using correct technique and lifting an appropriate amount of weight it should not cause problems.

Not necessarily: It depends on the types of lifts you are doing, your technique, how much weight you are doing and the underlying condition of your knees. As a general rule if you have "bad" knees, be careful about exercises that require deep knee bends, particularly far past a 90 degree bend between your lower leg and upper leg.