If this is from RMFRP Character Law, it says "this is an uncontrollable, psychotic type of lycanthropy. While you are in your animal form, you have a bestial desire to kill and destroy." That seems better balanced than the RMSS version which merely says -50 to all attempts to control it.
The RMFRP version I would treat as an NPC while in beast form, I would not even tell the other players right away that the thing attacking them is their fellow PC. I might tell them his bedroll has been shredded and there are signs he may have been dragged away.
The RMSS version is tougher. Simple answer is that I would not allow it. But if I did, I would spin plot around it very quickly. This is a flaw, not a talent, so it should be disadvantageous to the player. If they make it a net advantage, you are being too generous. So... who infected him with lycanthropy, and what are their motives? How do other lycanthropes feel about having the PC running around? Is he infringing on their territory? Disrupting their plans? Making them look bad? Did they cut him loose to serve some purpose? Do the greater lycanthropes (or the ones controlling them) have the ability to control him?
In either case, I would have the townsfolk in a tizzy as soon as they suspect, coming after him with pitchforks and torches -- but only in daylight when he's in human form. At night they stay in their homes and bar the doors and windows. If he causes any significant slaughter, I would have hunters hired to track him, with suitable weapons and knowledge. I would probably have someone oblivious try to hire the party to "hunt the terrible beast" and give them some of the info, as a warning to him.
And then I would have a way for him to quest for a cure (which would result in some replacement flaw, loss of a balancing talent, or some other cost; you don't get rid of flaws for free).