Why did Gan kill Dong Ruoyan?

In the TV series Passion between Heaven and Earth, Gan killed Dong Ruoyan because Dong Ruoyan's brother took his place in Yihai Group, which made him fall from the top of Yihai Group and was excluded. This dilemma led to the gradual distortion of his mentality and the deepening of his hatred for Dong Shi's brother and sister. Finally, he stepped into the abyss of crime step by step and killed Dong Ruoyan.