Each one of my characters are very different from each other. I don't personally feel guilt for any of their actions or choices, but I do actually feel pity and empathy for them at times. So in a way, I may be ashamed of them but not of myself for allowing them to act as the characters I've created naturally would. I'd like to think that we're all fairly well-balanced, reasonable people here, so most of us know better than to do/say certain things, but our characters don't necessarily have to be that way.
One character in particular of mine is pathetically naive, and I've put her through absolute hell over and over again. I don't feel like I'm sadistic for doing it, as I'm really just letting her live her fictional life the way her personality dictates. When other characters who know better than her drop in to give her guidance and point her in the right direction, I'm happy for her. When characters who know better drop in to take advantage and point her in the wrong direction, I certainly find it interesting, and I like seeing the way it plays out. I've cringed a few times when she's made the stupidest decisions that have had the biggest impact on her character, but I've had to allow it, of course, because if I had stopped it because I knew better than her, it would have been like...well, essentially divine intervention.
I have another character who was led astray at just the right (or wrong, really) point in her life, causing her to be a rather fragmented, lost person. I pity her from time to time, but I find myself feeling empathy for the characters she interacts with more often. It's kind of like what you said: sometimes I want to give the characters she interacts with some slack - especially when they appear to be experiencing great distress because of her behavior. Because of events that have altered her character over time, she can be unpredictable, offensive, and just downright horrible to other characters at time. I think I did slip in a tiny OOC note, a long time ago, to a couple of people I played with to apologize for her behavior, but nowadays, I just tell myself the players understand that the characters of Illarion are like the people that surround the players: they come in all kinds. If every character was stable, polite, poetic, handsome, and strong, Illarion would be so boring! I do know that some players are naturally inclined to avoid such characters as this one because we, as humans, realize that there's something very wrong with such a person (even if it is a character), and some players don't want to put their characters through what potentially harm them. Some players care very much for their character's well-being, of course, so they do everything in their power as players to avoid letting their characters suffer because of exposure to extraordinarily damaged people in their world. Chances are, if players are willing to allow their character(s) get hurt or otherwise influenced by your character, good or bad, they'll allow their characters to stick around yours more often. Because of that, I don't think any OOC apology or explanation of a character's behavior is really necessary.
Oh, and there's another character I play... She's not overwhelmingly naive or broken or anything, really. I haven't been playing her for too long, but I always find it interesting when I do play her. Maybe it's because I've played characters who have been greatly influenced by other characters already that I now appreciate just how much of an impact characters have on each other. Now, I see the characters she interacts with for who they are and what influence she may have on them and what influence they may have on her. Her potential actions with other characters present her with a million different, potential paths to take, and each one of them leads her to a new her - an influenced, still fundamentally her character, but altered in some way. And from that point? She can be further influenced! One character pointed out to her that she basically behaves more boldly in the presence of a certain other character, which she had been unaware of before. When she was next in the presence of the character who incited her to behave more boldly, she behaved somewhat self-consciously, unlike her normal behavior. This character in particular also seems to interact with a lot of opinionated characters who share their thoughts of other characters she interacts with, which in turn has occasionally affected her behavior around certain characters. Because the players of those characters are entirely unaware of what happened to alter my character's behavior around their character(s), I sometimes want to assure them OOCly that she has not just spontaneously changed, but I think I should give the players a bit more credit than that. Should everyone respect each player's ability to handle their characters appropriately, each player must assume that any given behavior presented by a character has its causes, and so it needs no OOC explanation. The characters being treated differently, however, may not understand the idea that all behavior has causes that have made it what it is. For those characters, I sometimes feel pity. I think their struggle with it is much like a real human's, though, so that's...interesting too.
I kinda feel like a sadistic scientist.
Sometimes I allow my characters to experience horrible things, and sometimes I allow my characters to cause other characters to experience horrible things. In allowing these things, I feel like I'm just allowing characters to live as they would if they existed naturally in a world remotely like ours, their existences dictated by their personalities and the influences upon them. In the past, I've apologized on a few occasions for my characters' behavior to other players OOCly, but nowadays I don't feel the need. I think that players understand that the behavior of characters has been caused by different factors, and characters do not always (sometimes, ever) behave in ways that we, the players, would be proud of if we behaved in such ways ourselves. I pity characters at times; I empathize with them. That said, I don't feel personal guilt for anything... except then I admit I feel like a sadistic scientist, because I think it's so interesting to see how characters' behaviors are influenced by other characters.