Toxic shamans are regarded as followers of their mentor spirits, just like their non-toxic counterparts. However, due to external influences like excessive pollution, or the mentality and will of their shaman, the influence a mentor spirit exerts can differ drastically from a typical shaman. A Dog shaman, for example, typically displays characteristics of loyalty and protectiveness towards those it associates with (its "pack", so to speak). A toxic Dog shaman, on the other hand, might act instead as vicious and treacherous, forgoing its associates to follow an agenda all his/her own.
This doesn't mean that the shaman is acting in opposition with its mentor spirit, but rather has adopted the mentor spirit's darker aspect. In the above example, the toxic Dog shaman is still acting characteristic of a dog, but now more akin to if the Dog spirit were rabid.