What should I do? I heard something really bad on the baby monitor. Should I call the police?

I know I am really bad for doing this, so please don't lecture me. My neighbors have an intercom system in their house, and I can hear everything they are saying in any room through my baby's monitor. I usually don't listen, but it even picks up their phone calls. I admit that sometimes I do listen to the drama when I am bored, which is really bad, but she is having an affair with her 10 yr old son's baseball coach. I heard them talking on the phone this morning and she told him and I quote "if only he would just die. You know he has a $500,000 life insurance policy." then they both laughed. OMG What should I do? If I call the police, how can I prove I heard it? What if something happens to her husband? Please.... Advice?