We know from the Gemara that an intermediate is forbidden according to the Torah. As the Rambam puts it, we must not worship or praise or pray to any being besides God himself in order that that being should be an intercessor between us and God. But that is better than believing in a bad person.
We find people that are not strictly Monotheists in the sense of the Rambam and yet believe in some tzadik and that seems to be helpful. And we find other people that are monotheistic and yet believe in some bad person and that seems to affect them also to become wicked.
From what I can tell this idea of belief in a tzadik [that is that it is important to find a true tzadik] is highly plausible. Even Litvaks try to find the most righteous Rosh Yeshiva to learn from.The truth is it is hard to argue with this premise.
That problem is -as many people already are aware- that once a person gets the idea that belief in a tzadik is important, he or she will most often attach themselves to some charlatan and plays the role of a tzadik with great expertise.
There are groups that I think are on the wrong path, and I am thinking it is usually because of some issue with their leader, rather that how monotheistic they are.
For example Muslims. It seems to me that the issue is that their false prophet was a bad man. Also I see people get involved in some charismatic leader that is teaching values that are highly questionable and that in fact seems to affect them to act in bad ways.