Toa operated by a code, one that said they did not kill their enemies. To do so would have made them no better than they were.1
Notes
The Toa code was not fully written out anywhere.2
Toa didn't believe in killing their foes unless they absolutely have to.3
Coming very close to killing a being but not following through did not violate the Toa code. Killing plant life, even intelligent plant life like the Morbuzakh, would likely not be a violation of the code. Killing Rahi like Visorak likely wouldn't either.4
Toa did not kill unless there was absolutely no other choice.5