"There's not a single witch or wizard who went bad who wasn't in Slytherin."

It is said that all Dark Wizards were in Slytherin (though it is untrue, as Peter Pettigrew was a Gryffindor and Quirinus Quirrell was a Ravenclaw), though how many were from this house before Tom Marvolo Riddle began recruiting Death Eaters there is unstated. As Slytherin was his own House, it was certainly most easy for him to recruit there.
It might also be possible (though unlikely) that the Sorting Hat simply sorts anyone with ill intentions into Slytherin, regardless of whether they possess cunning and ambition. Vincent Crabbe and Gregory Goyle were both incredibly stupid and did not have that clever, cunning mind, but were still sorted into Slytherin.