A while back a poster on redstate wrote an article entitled Liberalism's Blind Spot: The Cause of Our Moral Confusion, where the author attempted to get to the bottom of why those damned liberals support such obviously out of whack ideas. Read it if you wish, it got me thinking, but if you would rather just see a summary of the areas that my diary pertains to, then the general idea is we liberals have a 'moral blind spot,' specifically an utter mistrust of power. We tend to always side with the underdog, even when the underdog is not such a good person.
Liberals do exhibit this behavior, and I will go so far as to say that we do actually favor "bad guys" in some cases. However, this is not a 'moral blind spot'--it is a recognition of what causes immoral actions: advantage. Advantage and power go hand in hand, in several cases one leads to another.
Advantage (power) becomes a problem when the advantage holder enacts their own self-interests at the expense of other's interest. In other words, advantage is the opportunity to exercise greed and greedy ambitions. If power corrupts, and absolute power corrupts absolutely, then it is the opportunity to press one's greedy motivations through the advantages provided by said power that fuels corruption. These greed-filled power-advantages are what scare us liberals: the possibility for the advantage holder to exploit his position to the detriment of others.
Even though I've used the words interchangeably up to this point, an advantage does not always denote power, but power always denotes advantage, and certainly advantage must be exploited (to some extend or another, depending on the system in place and situation at hand) to gain previously un-guaranteed power. This would be the point where the underdog would have greedy motives of their own. They may have some advantage that they use to take power for nefarious means; although it should be noted that this is often a response to the powers-that-be already working against them. For instance, gang violence and gangs lashing out at police may be a result of those without power (the gang) using the resources available to them to fight back against those with power (the police, other gangs). However, said gang may increasingly abuse their newly formed organization for increasingly less noble goals--from running drugs, to selling weapons (which may at first be to provide a means to fight back but quickly becomes about profiting off of the violence itself), to simply protecting their limited power by any means necessary. The last point is significant because it demonstrates the point where the rebel becomes as corrupt as those he was rebelling against.
It is therefore a simplicity to say the underdog is always right, and admittedly the example I gave is an oversimplification of sorts, but the idea remains--Those without power are at the mercy of changing motives of those with power. It is not a blind spot of the liberal, it is recognition of the building blocks of inequality, oppression, and, ultimately, greed. That is why unchecked power especially bothers us, whether in politics, business practices, or our personal lives. That is why we scream and shout that the US military should exert extreme caution before crushing a weaker country, why we aren't quite so bothered by affirmative action, and why we consider ourselves champions of things like equal rights.