Have you or your parents tried speaking with him about his online behavior? It does sound concerning and threatening remarks aren't to be viewed lightly. It is understandable how heightened his emotions are. Separation and Divorce tear down self esteem regardless of who initiates. Anger is a normal feeling to have. His rejection triggers seem all over the place. Certainly one moment he's seeking some form of companionship probably to sooth that pain and on the other hand resentment turns to rage at the thought that the woman he once was married to could even move on.
If your aunt feels threatened and that her life could be in danger, it's up to her to decide how to proceed. From a personal standpoint, my ex did threaten me and whomever I had moved onto after 3 years divorced. But that wasn't what brought about a restraining order that was into its second year. He physically assaulted me, almost broke my elbow trying to grab a little tape recorder bruised me up good and forced me out of my car as it was the door frame that he had my arm against when the police came. People working at a business next door called. He ended up with probation, fine, anger management, some good friends that are in both of our circles to address himself. The restraining order came off via the family probate court, the judge felt that it would be beneficial as to make the shared legal custody work better and to create a co parenting situation that benefits the children's best interest. That with the understanding that another assault will result in jail time, plain and simple.