Li Fan wasn't sure if capturing Lu Chai alone would be enough to stop the catastrophe.
Preparations had reached the stage where weapons were being distributed, and any element of surprise had likely been accounted for in their plans.
Moreover, even if it were possible to prevent it, Li Fan personally wouldn't have chosen to do so based on his preferences.
He wasn't here to take sides. People inevitably hold double standards, but there's a limit to how hypocritical one can be. Why should only those in boots be allowed to strike others and not expect retaliation from those in straw sandals?
Of course, Lu Chai and his group weren't exactly innocent straw-sandal wearers, and perhaps behind them lay many of their own conspiracies and impure motives. But so what? Does that mean his armed peasants don't represent the oppressed underclass?