There's understanding and then there's understanding.
Take this for instance: http://www.achristianhome.com/to_train_up_a_child.htm
Which is basically about how to physical and mentally abuse your child into being a perfectly obedient drone. Now, even if this worked, I am absolutely incapable of understanding an empathizing with the sort of mind that would want that.
Now, I understand enough about that sort of person that I could probably press their buttons. I could maybe even fake writing a character that thought that way. But on a very real level, these people are aliens to me.
Their way of thinking and frame of reference is so different from mine that they become a black box: I can understand that if you input certain situations, you get a certain result, but the underlying thinking is something I'm not capable of getting.
Now, what this has to do with comics is that it is difficult, although not impossible, to write characters that are that different from what you think. I can write murderers and monsters even though I am not a monster or murderer because their actions are based in things that I can understand and empathize with enough to be able to get into their headspace.
I've gotten asked a few times whether any of my characters are like me, which doesn't seem like a hard question. All of my characters are like me, in the sense that their minds work like mine work at the base level. Their values and conclusions and actions are different, but the actual mental steps they take to get from point A to point B are definitely based in the way I think.
The analogy that fits best is that the characters I create all have the same base operating system but the programs loaded into them are different. This is, in and of itself, not a good or bad thing. It's just a fact.
Where it can be a problem is when you don't realize it. If I were to write, for some reason, a character that wants to train up a child, there's going to be a level of faking it. If I did this casually, the character will always read wrong to me. So I try to be conscious and aware of what the limits of my own ability to understand are.