He has already had a spat with Qais, with neither knowing the other's true identity. In the turn of events that follow, Qais's father dies at the hands of Tabrez. Qais in turn, avenges his father's murder by killing Tabrez.
He had a prior disagreement with Qais, but neither knew the other's true identity. The outcome led to Qais's father being slain by Tabrez, prompting Qais to exact vengeance by killing Tabrez in retribution.