He doesn't have to accept lady white snake so mercilessly to complete the task assigned by his superiors. He can even act as a bystander and guide to help lady white snake. However, he chose the most cruel way to force lady white snake, which is inhuman.
In this TV series, one explanation is that Fahai caught lady white snake instead of other monsters. 700 years ago, the white snake broke into the cave in Fahai and ate the ancestral elixir, so the two became enemies.
For White Snake, the reason why she came to steal the elixir can be traced back to 1000 years ago, when she almost died in the hands of snake catchers, that is, Fahai.
But from all indications, things are far from so simple. For Fahai, it can be defined as a good person or a bad person. Don't think about what the sea says about saving lives instead of building a seven-story tower. In fact, he just wants to climb it. In his view, there is no distinction between good and evil in the world. As long as justice is good for him, justice is nothing. He will do good or bad. For example, Wang is a villain, so no matter what, he hurt Xiaoqing. Besides, Fahai just can't see it.