He can join Jake in the "irredeemably kills innocents, but the show will want us to forget that in an episode or two" category. Scandal relies on shock value, like its characters suddenly acting like serial killers, but it also relies on short attention spans and audience amnesia. Just as Jake brutally murdered Cyrus' husband / one of Olivia's best friends, but soon afterward was quickly back to "dreamy boyfriend for Olivia" status, so too will Huck, in an episode or two, be back in the "funny scene where Huck says something cute" category. That the show might have a character take one of these violent actions, and then genuinely follow through on its implications, is highly unlikely.
I can at least see an argument for why Jake felt he had to kill James and the others -- they were threatening B613/President Grant's election theft/whatever with imminent public exposure, and if that happens one can reasonably believe the republic will fall, cats and dogs will be living together, mass hysteria. Besides, if it wasn't Jake, it would have been someone else from B613.
Huck killed Sue on the theory that one day she MIGHT talk and if she does talk, she would be a threat to Rosen's career, and if she's a threat to Rosen's career, then Rosen couldn't offer him immunity and if Rosen couldn't offer him immunity, he couldn't have one big happy family. Even putting aside how selfish that motive for killing was, it just wasn't legitimately logical. There's no guarantee that Sue would talk, or that she would talk about Rosen. Even if she were, there's nothing to say that it would be any time in the near future. Rosen would have plenty of time to get him an immunity deal before she were to talk. And even if he did not, there's always his replacement, or really the entirety of the Department of Justice, who could hammer out an immunity deal. And Huck should know better than most, it's not like Command is going to say, "Man, Huck's blowing the whistle on us so he can reunite with his family. If only we could employ the legion of assassins, torturers, bribers and so forth to put a stop to this. But he's got an immunity deal with the prosecutor, so I guess he and his family is untouchable."
Seems far better for him to have just threatened her and been like "I'm a scary man. If you breathe another word about all these men, it'll be your last."
On another front: I hate to sound/be sexist and shallow, but we're to believe that these 16 power players in D.C., where there's an army of young and hot women, slept with Sue because she was kinky and on a pro-kink website?
Also, hadn't Sue already sent the proposal if not the entire manuscript to publishers? I don't see how anyone could be confident that the only copies of it have been obtained. And wouldn't "Author of prospective sexy memoir about sexy times with power players found murdered" still have the same effect of giving publicity and credence to her account?
Seems like a horrible idea to bring all 16 people Sue slept with in a room together rather than approach them anonymously.
Also, doesn't Huck have more than 2 billion and counting from redirecting the B613 slush fund? Sue could have gotten paid off from just that.