Why did the Fire King finally kill Wei Yan?

Suck your strength. According to the synopsis of Fantasy City, the Fire King finally killed Wei Yan in order to absorb his vitality. Yan Yan has ten brothers. Brother Shuo Gang was killed by a mermaid, and the new knight was killed by a deep sacrifice. The other eight brothers guarded the ten-story tower and were killed by the mad fire king, sucking their life force. With a glorious ending, he died at the hands of his most respected father, the fire king. Castle and Sakura in Fantasy City are two princes of the Fantasy Snow Empire. Kassel was born indifferent to fame and fortune, and did not want to be imprisoned by the shackles of the throne, while Sakura Kong was ambitious and tried his best to sit on the last throne. In the end, Castle personally killed KINOMOTO SAKURA, whose hands were covered with blood, and became the new king of the Magic Snow Empire. One day, however, Castle learned of KINOMOTO SAKURA's good intentions in an accident. All he did was to let his favorite brother have a free life.