“He [Rabbi Yehoshua] used to say: a pious fool, a cunning evildoer, an ascetic woman and the wounds of the ascetics destroy the world” (Sotah 20a). Our sages understood that as humans we make mistakes, sin and are riddled with inconsistencies. It is for this reason teshuva, repentance, was created - even prior to the physical world itself, allowing us to grow from our mistakes. As our Sages note in a somewhat different context “G-d creates the cure before the sickness” (Megillah 13b).   
Our rabbis noted that even the adulterous may have many great merits (see here for further discussion). One can fail greatly in one area and excel in another. Such is the human condition. What our rabbis could not tolerate was religious hypocrisy; using religion as an excuse to not help or even to hurt others, for personal benefit, or to appear much more pious than one actually is. Sin is understandable. Using the Torah itself as the basis of sin is not. It is those whom the Mishna describes as “destroyers of the world”. 
This is all the more telling when we consider that this teaching of Rabbi Yehoshua is the concluding part of the Mishna whose primary focus is the drinking of the “bitter water” by the suspected adulterous, including her possible death right then and there in the Temple. Yet it is not the adulterer who is called the destroyer of the world. That appellation is reserved for “a pious fool, a cunning evildoer, an ascetic woman and the wounds of the ascetics”[1]
If adultery reflects a total breakdown of personal morality the “pious fool” reflects the opposite danger, that of excessive piety[2].  “What is a pious fool like? For example he sees a woman drowning in the river, and he says: 'It is improper for me to look upon her and rescue her'” (Sotah 21b). Piety is great but misplaced piety is deadly. 
The danger of misplaced piety is a common theme of those who are mevali olam, destroyers of the world. The Talmud presents seven possibilities to explain a rasha arum, a cunning evildoer, two of which reflect religious hypocrisy. “Rav Yoseph bar Chamah said in the name of Rav Sheshet: He who induces others to follow in his ways” (Sotah 21b). As Rashi explains he asks others to follow his example, yet his actions are done for show so that others will think he be much more pious that he actually is. Rav Soloveitchik noted that his grandfather Rav Chaim of Brisk had the same persona whether in public or in private[3]. What you see is what you get is not meant for computers alone. Rashi adds that this “leader” whose “inside is not like his outside” acts such so that “they will be not be exacting after him to check his abominations”. How sad and tragic how often it is those who supposedly lead who are the disgusting ones. 
This definition of rasha arum is followed by one given by Rav Huna: “He who is lenient with himselfand strict with others.” This is the natural result of pretending to be more pious than one really is; taking liberties for oneself that one would not allow for others. Such is the opposite of true piety, which is inward looking so that one is strict on oneself yet lenient for others. That would be true piety.
The theme of false piety continues with the third category of mevali olam, that of an isha prusha, an ascetic woman. This is defined as a bteulah tzilonit, a single girl who prays (Sotah 22a). This is a rather astonishing explanation to say the least, and the Gemara explains that only those who use prayer for personal aggrandizement are deserving of such a harsh description. As Rashi notes often one “prays” with great fervor so that people will think they are much more pious than they are and as was the case by the rasha arum, are hoping people will thus not examine their overall character. We can understand even as we disapprove of those who do not pray. But to pray so that one can impress others upends the order of the world[4]
And finally we come to the “wounds of the ascetic.” The Gemara gives one case after another of those who injure themselves in order to feign religiosity but are none other than hypocrites. They walk in public with closed eyes lest they see a woman, banging their heads against walls so that blood flows; or walk hunched over or so slowly that they scrape their feet on the ground as acts of humility (Sotah 22b). As the boy who cried wolf once too often, the Gemara even suggests that actual mitzvoth done by such ascetics may be tainted due to using the Torah for their own benefit. 
Yet when all is said and done only G-d knows our true motivations[5]. “Rav Nachman bar Yitzchak said: What is hidden is hidden, and what is revealed is revealed; the Great Tribunal will exact punishment from those who wrap themselves in their tallit [as a display of false humility (Rashi)]. 
The third chapter of Sotah - which ostensibly focuses on the ordeal of the sotah - ends with the teaching of Yannai Hamelech, one who identified with the Sadducees in their historic struggle with the Pharisees. His insight “spoken to his wife” is something well worth listening to. “Fear not the Pharisees and the non-Pharisees [Saducees] but the hypocrites who ape the Pharisees; because their deeds are the deeds of Zimri but they expect a reward like Pinchas”. The greatest threat to Judaism comes not from the non-religious but from the “religious” hypocrites who twist religion for their personal gain. 
[1] While many (though not all) of the activities that “destroy the world” are objectively speaking of much lesser severity than adultery our sages viewed these as even more dangerous. It is possible that the occurrence of adultery was relatively rare in Talmudic times (though that is speculation on my part) allowing the rabbis to focus on other sins which were much more prevalent. The Torah does warn that sexual immorality will drive us from the land. Presumably that is when such sins are rampant.
[2] The Tosafists quote the example of the Jerusalem Talmud of someone who is busy in prayer and insists on removing his tefillin before offering to help. As misguided and terrible as the one who does not want to touch a woman is, at least there is, under normal circumstances, a prohibition involved. To let someone die as they pray renders those prayers worse than meaningless.
[3] The Rav also noted that it is not necessarily hypocrisy to act in a slightly different manner in public than private. We are influenced, or might we say motivated by our environment. (I would greatly appreciate if someone could send me the written source, which i cannot recall.)
[4] Why the Gemara singles out a single woman when men can also pray for self-aggrandizement is not clear. At the risk of being politically incorrect perhaps it is because those women who pray tend to do so with greater fervor than men. While we hope they are sincere such is not always the case.
[5] Often we do not fully understand our own motivations. It is thus best to avoid ascribing motivation to others.