"Muirne, have you really gone mad? Stop this at once! You'll destroy Britain!" Muirne's actions made Boggart struggle desperately. Whether for public or personal reasons, he could not allow the Fang Clan Goblins to be wiped out here. Unfortunately, grievously wounded as he was, he had no way to break free from his restraints. After all, he was currently within Muirne's Fae Domain.
As the last member of the Wing Clan, Muirne might not have been particularly strong in a direct battle, but her Fae Domain was extraordinarily powerful. In all of Fae Country, only a handful of people could ignore it. Had Boggart been at full strength, he might have been able to escape Gloucester through sheer power, but he no longer possessed that strength.
The moment the Fang Clan Goblins entered Gloucester, their fate had already been sealed. The entire city lay beneath Muirne's domain. She had begun making preparations a thousand years ago, showing just how fierce her desire for revenge was. As long as she could have her revenge, she did not care about any cost. It was not as though she did not understand the Fang Clan's importance to Britain, but for revenge, none of that mattered. Perhaps revenge was her very purpose as a Goblin.
This also caused her mind to become unstable after her revenge succeeded. She selectively forgot certain things, to the point that she later forgot she had slaughtered the Fang Clan and instead sent her subordinates to search for them.
There was nothing wrong with Muirne seeking revenge against the Fang Clan; even Koyanskaya acknowledged that. But she had chosen the wrong time. She could have waited until later to act. That had been Koyanskaya's thought as well, but Muirne could not restrain herself and killed every Fang Clan member.