I think kakashi realy held back when he was the leader/teacher/trainer of the original team 7. Whats the point in him going out and killing off all of the enemies the group fought before the kids had a chance to learn anything?
Sasuke would have been killed if Haku wanted him dead. They can't learn anything if they are dead, so what's the point in holding back if your students die.

Kakashi was not holding anything back agaisnt Zabuza, he even used Chidori then and he had no chakra left to do anything other than bluff with a kage-bunshin when Gato and his goons showed up.