If one were to assume, as some people say, that Yazid did not issue orders to kill Imam al-Husayn (as) and that Imam al-Husayn (as) was in fact killed as a result of ‘Ubayd Allah ibn Ziyad's independent actions and that Yazid was displeased by Ibn Ziyad's deeds, Yazid should have confronted him strongly with intense opposition to his insubordinate behavior. He ought to have reproached him in a way that suited his appalling act.
Even in the case that Yazid was opposed to Imam al-Husayn (as), if he in fact did not issue orders to kill the Imam and ‘Ubayd Allah ibn Ziyad committed this hideous crime without Yazid's authorization, he should have been sharply censured for overstepping his authority and defying the central government. However, what historians have recorded is that not only Ibn Ziyad was not rebuked by Yazid in the slightest, but was instead rewarded handsomely on many occasions.
Yazid's unanticipated behavior under the circumstances casts grave doubt on the veracity of the idea that Ibn Ziyad was acting independently rather than under direct orders from Yazid ibn Mu‘awiyah.
Adapted from: "The Uprising of Ashura and Responses to Doubts" by: "‘Ali Asghar Ridwani"
source : www.rafed.net