I am pretty violent in lucidndreams, but normally violence for a purpose is what I do. Not violence for fun, unless it is anime style. But we all know that anime style doesn't kill people, you just turn them to your side.
In Sword Art Online Kirito said something to the effect of
"It is more important not to do evil in here because it is all just in our heads, and it may not effect the people around us, but it effects us." Definitely not even close to perfect on the quote. But the idea is there.
If you go around chopping off heads and killing younglings (he said that you killed younglings!) that has to have some psychological repercussions. This would be similar to the person that sits around all day and thinks about shooting up a school.
Now I am not saying that all killing of DCs is wrong. I kill DCs a lot actually.
I make a quest and the DC in my way is a "bad guy" or an orc or a Sith. I can't see anything wrong with wanting to kill one of these types of people. One time my "dad" in dream told me to do something that would show how much I knew that it was a dream and that it was all in my head. He teleported me away and I jumped a DC and he didn't fight. He just smiled at me as I snapped his neck. Honestly, a horrible experience.
I see nothing wrong with this though, because I an not trying to live out my fantasy of killing people, I am trying to understand the truth, there is no DC. I would recommend things like this for better dream control. Things that you could do in waking life, but you don't. Take off your clothes in public, jump in front of a car, kill a DC, have sex in public (you are probably gonna have sex with a DC anyways
), anything that you can think of along those lines.
Raping DCs. I have never felt the need to rape DCs, mostly because every DC girl jumps me and tries to have sex with me as soon as I realize I am dreaming. The exceptions are the ones that I like to have on adventures. Get a real fake relationship going. Haha. I think that fantasizing about doing things like this could lead to acting it out. Most rapists do say "it began with porn, and then it just kept getting to worse and worse porn." It is the rule of diminishing returns, you never will be satisfied with what you have (appetite wise food, drink, sex) you will always want more, so be careful with that you are wanting, it could cause you troubles.
So in conclusion, it isn't what you are doing, it is why you are doing it.