Why did Lord Shiva kill Surya Dev?

Why did Lord Shiva kill Surya Dev? Once, the son of Rishi Kashyap, Surya Dev killed Shiva’s devotees named Mali and Sumali. Angry, Lord Shiva killed Surya with his Trishool. Once Siva in great anger killed Surya with the trident who had killed the demons named Mali and Sumali. The trident was full of lustre like vajra and infallible. With the striking of the trident, the sun fell down from chariot and fainted. Brahma Vaivarta Purana, Ganpati Khanda, Chapter 18. Angry, Rishi Kashyap cursed Lord Shiva to kill his own son with his Trident: The gods also felt terrified and contributed to the grief of Kasyapa. At that point of time the entire universe was plunged in darkness. Kasyapa who happens to be the grandson of great tapas Brahma and was illumining with divine lustre. Finding his son falling pronounced a curse on ^iva that the way in which you have killed my son similarly the head of your son will also be shattered. There is no doubt about it. Brahma Vaivarta Purana, Ganpati Khanda, Chapter 18. That is the reason why Lord Shiva beheaded his son, Ganesha with his Trishool.