as salam alaikum
different sources depict Shimr Ibn Dhil-Jawshan and Sinan Ibn Anas as the killers of Husayn, peace be upon him. It is evident that both played a role in the killing although there may be some variations in the details of various reports. Both were fighting along the army of 'Umar Ibn Sa'd, sent by 'Ubaydullah Ibn Zayd, appointed by Yazid Ibn Mu'awiyah.
With prayers for your success.