Let's take a look at XinXin Zhai's case first, which is nothing more than two points: first, Su Xiangmao committed suicide; Second, XinXin Zhai took advantage of Su Xiangmao's privacy and the divorce agreement.
Let's look at the first point. Su Xiangmao committed suicide, not XinXin Zhai. You would also say that if XinXin Zhai hadn't pushed Su Xiangmao too hard, he wouldn't have committed suicide. This is only a causal relationship in life, and it cannot be a causal relationship in criminal law.
Second, XinXin Zhai and Su Xiangmao signed a divorce agreement. XinXin Zhai seized Su Xiangmao's privacy and forced Su Xiangmao to sign a very unfavorable divorce agreement. Many people think this is unreasonable, but the premise is to sign a divorce agreement.
If you want to sue XinXin Zhai for extortion, you must prove that XinXin Zhai planned for a long time before getting married and covered up his illegal purpose in a legal form. If this can't be proved, it is estimated that it will be a bit difficult to implement.
In fact, Su Xiangmao had too many chances to make a comeback in this incident, but he lost his temper. He can consult his lawyer about the legality of the agreement when he divorces, and he won't end up like this.
If the criminal case fails, you can try to take the civil route. Su Xiangmao's parents can also sue XinXin Zhai and cancel the divorce agreement. It is estimated that there is still a chance to get back some property.