Kato is definitely just putting Igarashi in the same position as the twin kunoichi. Igarashi's likely itching to kill some women, and the twins want to kill him as revenge for skinning Hime-chan. Kato stays uninvolved either way.
I'm sure Kato isn't exactly thrilled to have the "crazed serial killer" Igarashi as a confidant, plus Igarashi was beaten before by Juro and the others at Ekoeko Tower. If he isn't strong enough to ambush and kill these two "elite kunoichi", maybe Kato thinks that he'd be better off working alone?