Why wasn’t the Atreides slave drugged?

Reading Dune for the second time and have just finished the arena fight.

I’m aware that Thufir Hawat and Feyd-Rautha worked together to make sure the Atreides slave wasn’t drugged in order to frame the slave master and have him killed, but why? Not sure if I’ve missed something or if it just doesn’t explain, but I don’t understand the motive behind this.