Evil is way too subjective to say "this group or that group doesn't understand evil". Evil is, for the most part, about perception.
Hitler thought the Jews were evil. Catholics thought Protestants/Muslims/Jews were evil during the crusades/inquisitions/etc. Romans thought Christians were evil. This goes on and on. The real problem is that people are
given perception from another, more well spoken, charismatic, leader, be it religious, political or otherwise. For the most part, since I didn't actually read the link provided by the OP, people are not evil. Do you think all the Germans fighting for the Nazi party wanted to annihilate entire races? Do you think all Catholics want to see Protestants hung as traitors to the christian beliefs? Did everyone in Saddam's regime enjoy torturing and killing their countrymen? Not all terrorists are evil, most are doing what they believe is correct in the eyes of their god, with some coaxing from people that could probably be construed as evil.
This is a big reason I am not a big fan of organized religion, too much power in one institution also breeds evil. People become so self-righteous that they believe their word is like that of god and they become untouchable, or that their interpretation of the bible is the right one. It's absolutely incredible what some people will believe when you deliver it the right way and this is where most "evil" is born.
To answer the OP, "liberals" recognize evil the same way as "conservatives" and "moderates", through their own eyes. I believe a CEO taking millions in bonuses while laying off thousands of workers of a failing company is evil and immoral, but I know there are quite a few CEOs that would disagree.
Perception, as I see it
, is where we all go wrong, no matter the situation. Just ask your neighbor how he sees you, and, if they are honest, you might be surprised by the response.
Mike