Well, to all the people who think that logic of this anime is absurd then why do you even watch anime....its clearly stated that genre of the anime is gory , science fiction and dark fantasy. The creators must appreciated because they give exactly what an anime of this genre must give.
Now, to answer "how can you love someone who killed your sister and father?". Well, try to put yourself in the shoes of kohta...he was a kid, was in shock and trauma from deaths of his family, and still instead of hating he understood Lucy's suffering and made amend with it.
It is one of the most touching and heart wrenching anime.... its so good that I'm gonna ignore all the other technical errors and just give it straight 10 out of 10.
This is a very good anime but if you have any and I mean ANY reservations to mature content then you may have a hard time liking it. It is very brutal, disturbing and violent. However at the same time the series has enough soft moments to balance out the constant vicious blood and gore. The characters are very well developed (which makes it all the more heart wrenching when some of them get ripped to pieces) and the pacing is very good. The show also has several thought provoking themes about isolation and bullying, It just has a very in your face way of showing them. Overall a masterful but very visceral anime that only the hardcore should watch.
For those who don’t want to read the rest of my review for whatever reason, I just want to say that I recommend watching Elfen Lied.

People dislike the show because of the gore and nudity in the show. I personally would say the English dub wasn’t the best either. I bought the entire show on disc and watched it again in Japanese. It was a lot better than I remembered. People who reviewed the slow and complained about the gore and/or nudity probably shouldn’t have watched it in the first place. The story is supposed to be about how cruel the world can be to people who are considered different. Considering how it’s an anime from 2004, made by studio not well known (at least at the time it aired) and crammed into 12 episodes (and one OVA) I am surprised it came out this well. It’s definitely a good anime and worth a watch.
I don't think we should just judge this great anime just with the thing we saw.
You know lots of character are kinda symbolic and it must not be judged with first watch. Cause it may not show what it really got in hands!
The whole atmosphere is some how very bright and cute but also you can feel the darkness !
If see some comments about why this man gets erection most of the time and there are 2 very simple answer for that;
first of all he is a teen and second some things that are not taboo in your country might be in other's country.
in conclusion, don't die before watching this great anime!