Kiri-sute gomen (斬捨御免 or 切捨御免) is an old Japanese expression dating back to the feudal era right to strike (right of samurai to kill commoners for perceived affronts). Samurai had the right to strike with sword at anyone of a lower class who compromised their honour.

What bad things did samurai do?

The samurai often participated in ruthless roadside killings

Though the practice had its roots in honorable dueling, it eventually twisted into a popular practice where members of the samurai class sneaked around to unexpectedly slash at passing merchants and peasants.

Who was the deadliest samurai?

Born in 1490, Tsukahara Bokuden is one of the most prominent figures in samurai history. Over the course of 19 duels and 37 battles, Bokuden went completely undefeated, garnering a reputation as the most deadly samurai during the Warring States Period.