They base their claims on saying that companions such as Sulayman b. Surad and Rifa'a b. Shaddad wrote to Imam Hussain (as) inviting him to Kufa to be their Imam, but then deserted him at the last minute. They also cite the Tawwabun uprising as "proof" that the Shi'a deserted Imam Hussain (as).
This baseless claim is made by those who try to deny the facts of history admitted by their main scholars, just to run away from the curse on their leaders who caused the killing of Imam Husain (AS).
Let us go briefly through the crime and who committed it to know whether Shia or their opponents killed Imam Husain (AS):
1. Imam Husain (AS) was killed by Yazeed son of Mo’awiyah, who sent a big army to Karbala for this crime. The army was led by Omar Ibn Sa’d.
Mo’awiyah is praised by many Sunni persons claiming that he was a companion. They overlook his crimes which are clear for every student or researcher.
Shia curse Mo’awiyah because he fought against Ameerul Mo’mineen Ali ( AS), while many Sunnis praise him.
2. Yazeed is cursed by Shia while you find some Sunni books praise him.
3. Omar Ibn Sa’d is cursed by Shia because of his role in killing Imam Husain (AS) while you find big Sunni scholars like Ibn Hajar al-Asqalani praising him and claiming that he was an authentic narrator of Hadeeth ( Taqreeb al-Tahtheeb, 1:717).تقريب التهذيب لابن حجر
Another Sunni book by the name of Ma’rifat al-Thiqaat by Al-Ijli معرفة الثقات للعجلي admitted his role is killing Imam Husain, yet praised him 2:166.
Al-Dhahabi is another very prominent Sunni scholar, praise Omar Ibn Sa’d in his book Meezan al-I’tidaal 3:198. ميزان الاعتدال للذهبي
So, when you see these facts and many similar facts, How can any person with minimum sense accept the false allegation that Shia were behind the killing of Imam Husain ( AS) ?