It's worth a read, even if you dislike the anime. The manga explores a bit more moral questions, nature vs. nurture, and how much can we blame someone heavily traumatized for his actions; the anime doesn't really explore those points, it's IMO just a boring bloodfest (the opening song is amazing though).
Don't expect the manga to be a masterwork though.
I don't know if it's intentional from the author's part, but the Diclonii can be see as an analogy for certain gifted children. Even the relationship between Lucy and Kouta can be seen this way.
Speaking on Kouta: yes, he's a weak-willed imbecile, but there's a good in-universe reason for that.