Can I sue a bar for negligence because its secuirity let the patron that attacked me be go?

You may have a case. The question is whether you can find a lawyer willing to take the case, either on a contingent fee basis or for whatever you're willing and able to pay. That's one threshold question.  The next is whether the bar has enough money (or enough insurance coverage) to make it worth your while to go after them. The fact that security personnel just let your assailant go doesn't really help you at all. They're not the police. The real question is whether the bar owed you as a patron a duty of care to prevent the injuries you suffered. Could security personnel reasonably have foreseen that your assailant was going to be a problem? Did they have a duty to remove your assailant from the bar before the incident took place? That's not very clear, is it?  At least from your brief account of the incident. The economics of a lawsuit are such, however, that it may be in the best interests of the bar (or the bar's insurance company) to pay you for your medical bills in settlement of a doubtful claim. The adverse publicity may cost them more than your medical bills. To protect your position, however, don't you think it would be wise to file a police report and press criminal charges against your assailant? If you do sue the club, your attorney will probably recommend suing your assailant too. You can't just go after the deep pocket when the proximate cause of your injuries was your assailant, not necessarily security personnel who might have prevented your injuries if they'd identified your assailant as a problem before he assaulted you.