If I ever talked to a Toa about how some masks are immoral due to the power, I would ask them one or more question on the mask in question here are some examples:

Jutlin aka Mask of Corruption (not a very fitting alternate name IMHO):
Does a metal object not corrode?
Does a organic being that is no longer alive not decompose?

Crast aka Mask of Repulsion:
Does a being not want to seek shelter from falling rain and snow?
Does a being not want to keep itself clean?

Mask of Aging:
Do things not age?

What would you say to the Toa?

You know, technically, Onewa's mask power is that of a Degree of mind control, or suggestion.....not to mention Vakama's invisibility. and Matau's shapeshifting...

6591035
think it was more cause the makuta vs the power

6591188 Yes but their powers are aren't considered immoral mask powers

6591284 The Shelek's mask power is not considered an immoral mask power by the Toa, they just usually don't wear it due to it's association with the Makuta, also the Mask of Mutation's power is not considered immoral mask power as the only known (named) wearer of it (Makuta Miserex) did not wear it however only Makuta (as there were numerous unnamed Makuta that wore it before they were killed) have been known to wear it.

6591035
A lot of things that are generally skimmed over and forgotten about with regards to the why's of the Toa philosophies and views are explained by the reverence of Mata Nui that the Toa hold for him, the reverence itself is also quite frequently glossed over and forgotten about as well. Those masks that are seen as immoral are seen as immoral because they alter Mata Nui's creation in an unambiguously negative way or the concept that that kanohi embodies is in contrast to the three values of Mata Nui or his ideals. Toa are less like the "elemental warrior" that people confuse them with and more like a Paladin in that they are religious warriors acting as guardians and defenders to the faithful Matoran. There cannot be more emphasis on just how much the Toa value and revere the Great Spirit.