Is it a bad idea to contact someone in prison who assaulted me?
In most cases, yes. Here is why.
The dynamic between a survivor and the person who harmed them rarely plays out the way the survivor hopes when contact is reopened. You may be looking for an acknowledgment, an apology, an explanation, or just some version of closure that feels like it will help you move forward. Those are completely understandable things to want. The problem is that you cannot control what he does with the opportunity you give him. He may deny it. He may minimize it. He may say something that makes you feel worse than you do right now, and you will have no way to prepare for that in advance.
There is also a practical consideration. If there are pending charges, an open investigation, or any possibility of civil action related to the assault, contacting him directly could complicate your position. Anything said in that exchange, including your words, can surface later in ways you did not intend.
If what you are genuinely seeking is closure or answers, there are structured ways to pursue that without putting yourself in a vulnerable position. Victim-offender mediation programs exist specifically for this purpose. They involve a trained mediator, happen on your terms, and give you a framework that a cold letter or phone call does not. Your state's victim services office can tell you whether that option is available for your situation.
If there is more to the story that you want to talk through, share what you are comfortable sharing. Sometimes talking it out helps clarify what you actually need before deciding whether any contact is worth the risk.