 The overlying reason for this is that all Machinas are incapable of intentionally killing another human on their own volition; they need a "Factor" i.e.another human, who would be the one who "kills".The Factor need not be actively piloting the machina for it to kill, as revealed when a rampaging Machina called Apparition was brought down by JUDA and the then-unaligned Hayase and it was found that the (dead) Factor was drugged and restrained inside it.