When it comes to Nena, I do pity her somewhat. Considering she was grown in a test tube and raised to be a violent killing machine by Ribbons, it was almost a given that she became such a selfish and cruel person. Some people rise above their past, and some fall even further. Nena was in the latter category, and was never going to do anything with her life except exploit and murder people for her own satisfaction, so her death was pretty much necessary for the good of everyone else.
Also, it speaks volumes about Louise's character that while she had a perfectly legitimate reason for killing Nena, she took no pleasure in the task and actually showed some regret immediately afterwarRAB. Louise has her faults, but despite her loyalty to the A-Laws, she constanly shows she still tries to keep her morals in a bleak situation. The contrast between her flawed but noble struggle versus that of Nena's totally self-centered existence was pretty clear during that episode.
Other notes:
Graham's really gone off the deep end. He really thinks he's some sort of samurai. I liked him more when he was the All-American Ace. At least he seems to be coming to his senses now
Wang, we hardly knew thee... seriously her death felt forced. It was nice to see that she was human underneath that hardened exterior, but I feel like the writers could have done more with her character. I understand now what her motivations were, and they were interesting to say the least, particularly within the context of the series. Killing her off five minutes later seemed hasty. Plus it negates Hong Long's noble sacrifice. The guy went out of his way to protect his sister, and it was rendered meaningless. Stuff like that happens in real life too, but I don't think that was the intent here.
One other thing. Hong Long was shown in earlier parts of the series to be very intelligent and quite strong and capable. Wang Liu-mei implied his parents thought him deficient in ability to lead to Wang family, but he showed no signs of that on screen. I think that was somewhat of a furable on the part of the writers as well.