First, Ximen Qing is rich and powerful.
Before Ximen Qing murdered Wu Dalang, he knew that Wu Dalang's younger brother Song Wu was fierce and famous for fighting. But Ximen Qing is rich and powerful. In that society at that time, the rich were not afraid of a warrior.
He thought that after he killed Wu Dalang, he could use money to dredge the relationship in the yamen. Even if Wu Erlang found out that he killed Wu Dalang himself, he couldn't help it.
And he didn't do it himself, just let Pan Jinlian and Wang Ma strangle Wu Dalang. Ximen Qing's idea at that time was that even if it was exposed, it had nothing to do with him, and he could give himself a push.
Second, Ximen Qing was carried away by love.
As a businessman, Ximen Qing has a clear head and can analyze things rationally. However, his act of hurting Wu Dalang is very thoughtless, mainly because he is very eager to get Pan Jinlian and wants to marry him.
And he thinks that Wu Dalang is not worthy of Pan Jinlian at all, and he is helping Pan Jinlian by doing so. He was carried away by love, so what he did was very irrational.
Third, Takejiro doesn't play cards according to common sense.
Even if Ximen Qing and the Queen Mother calculated it carefully, they never thought that Wu Erlang was a man with nothing to hide. Wu Dalang is Wu Erlang's only relative. After his brother died, he was very angry. Jiro was not a soldier. After seeing his brother's body, he began a step-by-step investigation and finally learned the real cause of his brother's death.
Song Wu and his brother have deep feelings. Of course, he wants to avenge his brother.
Summary: Ximen Qing knew that WuErlang would retaliate, but he thought that WuErlang would go to court with himself. Ximen Qing thought he could win the lawsuit by spending some money, but he didn't expect Wu Erlang to have such deep feelings for Wu Dalang that he committed suicide by going directly to the Yuanyang Building.