Can I file suit for negligent retention if my company forces me to continue employment of an abusive employee?

My employee slapped another across the face. I immediately asked HR to have the employee who committed the battery terminated. They (human resources) declined stating she was good with customers. I then asked to have her suspended, with pay, pending a full investigation. Again, they refused and allowed her to stay "on the clock". This situation has occurred more than once. Now, my employees want to know why "I" am tolerating a physically abusive worker to continue working. I know, as a seasoned director, I must exercise due diligence in preventing a hostile work environment. Additionally, I know the legal (and possible monetary) ramifications/penalties for negligent retention but my HR department will not back me. Subsequently, I have resigned from my position as I have now lost all credibility with my employees. I have written statements from 3 witnesses, the person on which the battery was inflicted and, if you can believe this, I even have a written statement from the person committing the battery admitting to it! I can not believe this would not be a "slam dunk" as a negligent retention case. Do attorneys ever take this type of case on contingency?