8. "Doctor, You Can Stop Now!"
Again, the Doctor commits genocide. Again, the companion makes him stop. This moment wouldn't have carried so much weight if it wasn't for 'Turn Left', but the scene where Ten kills the Racnoss places him squarely as a villain. Mainly, because there isn't much conflict in the Doctor's mind; he offers the Racnoss an ultimatum, to leave for another planet, and obviously, it's refused. But the Doctor then floods the pit, and won't leave until Donna tells him to. He kills the offspring of the Racnoss, and won't stop; he has so little compassion left after losing Rose. Compare it to two series later where he offers Davros help. This is why companions are needed in Doctor Who.