The Shia are with Hussein so they did not kill him. The word "shia" literally means group and in this case group of Ali, the Prophet Mohammad's cousin.This is the story of Al-Hussein:
Six months after Hassan ibn Ali(A.S)(the second Shi’ah Imam) was born; Fatima al-Zahra(A.S) binte Muhammad(S.A.W) became pregnant with her second child. Fatima al-Zahra(A.S) binte Muhammad(S.A.W) started noticing the signs that childbearing was near, but Muhammad(S.A.W) had already foretold of Hussain ibn Ali’s (A.S)birth.On the third of the blessed month of Sha’ban, the fourth year after Hijra, Muhammad(S.A.W) was given news of the birth of Hussain ibn Ali(A.S). Muhammad (S.A.W) hurried to the house of Ali ibn Abu Talib (A.S)and Fatima al-Zahra binte Muhammad(A.S).Saffiyah bint ‘Abd al-Muttalib, Asma binte Umais, and Umm-e-Salama were present when Hussain ibn Ali (A.S)was born.When Muhammad (S.A.W) asked Safia binte Abdul Muttalib (his aunt) to bring him the newborn child, she said: "We have not cleaned him yet". When Muhammad(S.A.W) heard this, he said: "You clean him? Surely Allah the Exalted has cleaned and purified him".Asma binte Umais took the newborn child to him. The infant was wrapped in a piece of cloth. The face of Muhammad (S.A.W)lit up upon seeing his grandson. He took him in his arms. He recited the call to prayer (Adhan) into his right ear, and read the shorter version (Iqama) in his left ear. He, then, placed the baby in his lap and wept.
"May my father and mother be your sacrifice", Asma binte Umais asked Prophet(S.A.W) , "why are you crying"?
"Because of my son", he(S.A.W) replied.
"He is a newborn baby", she said.
"O Asma", he (S.A.W) said, "After me, the transgressing party will kill him. May Allah never grant them my intercession".
Then he (S.A.W) said: "Asma, do not tell Fatima(A.S) about this, for she has just given birth to him".
After Hussain ibn Ali was born, Gabriel(A.S) descended to Muhammad(S.A.W) and revealed to him to give the new baby the name Al-Hussain. Al-Hussain is the Arabic version of the old Hebrew name Shabir, which was Harun’s second son’s name. When Gabriel(A.S) descended to Muhammad(S.A.W) , scores of angels accompanied him to congratulate and console Muhammad (S.A.W) for Hussain ibn Ali's (A.S)birth and expected martyrdom.
Seven days after the birth Muhammad (S.A.W) shaved Hussain ibn Ali’s head and gave the weight of his hair as charity for him.
Battle of Karbala
The Imam Husayn Shrine in Karbala, IraqMain article: Battle of Karbala
According to historians, Yazid had heard of the incipient rebellion in Kufa and sent a force to suppress the insurgents. Before Husayn even neared Kufa, his supporters had been crushed. A force of about 4,000 surrounded Husayn and his family (friends, women and children), cut off his camp from their water supply. After a long and hard-fought battle, Husayn and his warriors (said to number only seventy-two) were killed.
Shi'a views of Husayn
All Shi'as regard Husayn as an Imam and a martyr. Most accounts state that he knew that his efforts were doomed, but set out on his path towards martyrdom in order to save Islam and the Ummah from annihilation at the hands of Yazid. According to Shi'a belief he was a willing sacrifice to religious necessity, and Shi'ites view Husayn as an exemplar of courage and resistance against tyranny. Ashura, a day of mourning, is held in honor of his sacrifice. The Shi'a saying, "Every day is Ashura, every land is Karbala," is a reminder to live one's life as Husayn did on Ashura, with total sacrifice to Allah and for others.
This is the true story of Al-Hussein, the hero and martyr of Islam who preserved it for years to come. Take care.