?Why did Yazid order his soldiers to kill Imam Hussain

I was wondering, why did Yazid order his soldiers to kill Imam Hussien (May Allah be pleased with him)? Did he not know that Imam Hussain (May Allah be pleased with him) was actually the grandson of the Prophet (Peace and blessings be upon him)? If so, why he did he order his soldiers to kill Imam Hussain(a.s)?