Is running on a treadmill bad for me if I tore my ACL a year ago?

Not necessarily. As long as you have rehabed the musclesof your thighs (quads and hamstrings) running should not be a problem. Elliptical machines and bicycles may feel better though. If choosing to cycle, make sure the seat is adjusted propely - leg should almost fully extend when pedal is down, but the knee should not ever lock.
ACL Tear. Running on a treadmill in a straight line should be okay with an acl injury. If there are other injuries in the knee from when the acl was torn, these may be worsened by running.