YOU SAID:

the man sat on the cat which sat on the mat and killed it

INTO JAPANESE

男はマットの上に座ってそれを殺した猫に座った

BACK INTO ENGLISH

The man sat on the mat and sat on the cat that killed it

INTO JAPANESE

男はマットの上に座って、それを殺した猫の上に座った

BACK INTO ENGLISH

The man sat on the mat and sat on the cat that killed it