The identity of the beast having been revealed in "Teen Wolf" season 5 does not make it easier for Scott and the others to bring the bloody war in Beacon Hills to an end. With the group now aware that they are up against their close friend Mason, things just got a lot more complicated.
In "Teen Wolf" season 5 episode 19 titled "The Beast of Beacon Hills," the pack is compelled to take on someone they love and they have no choice but to take him down seeing that he can kill and hurt more innocent people if they don't stop him. Of course, the group hopes that there is a way to defeat the monster without hurting Mason.

This is the hope of Liam as well. Mason is very close to him and has in fact become best friends with him. "Teen Wolf" has showcased many times how far their friendship has gone. Ever since Mason learned of Liam's werewolf alter ego, this made them closer than ever.
He got himself involved more and more in the world of supernatural and offered services to wolves whenever he could. However, in "Teen Wolf" season 5 episode 19, it does not look like Liam will allow his interests to get in the way of keeping Beacon Hills safe.
Nevertheless, it does not look like he will give up on his friend. It is still unclear whether or not Mason is aware that he is the beast. It would be much more difficult to pull him out of the creature's control if that were the case.
Right now, Scott and the gang are focused on halting the Dread Doctors in their tracks. Their final plan against the wolves is about to unfold so the pack is set to race against the clock to thwart the bad guys before they put their atrocious plan in operation.
Whether or not they will be able to stop them remains up in the air. "Teen Wolf" season 5 episode 19 "The Beast of Beacon Hills" airs March 1 at 9 p.m. ET on MTV.