The Blackbriars and Silver-Bloods are not respected in their respective holds. They are extremely feared and practically control the Jarl's seat.
Also Maven has extreme ties with the Dark Brotherhood which means she is probably responsible for a lot of murders.
Morally wrong to kill them? Maybe it is, but it may not be legally wrong considering this is Skyrim.
Listen to Ungrien's dialogue next time.
Also there is Mjoll the Lioness . . .
Mjoll: "I had another run-in with the Thieves Guild." Aerin: "Be careful, Mjoll. The Thieves Guild has http://uesp.net/wiki/Skyrim:Maven_Black-Briar at her back. One snap of her fingers, and you could end up in http://uesp.net/wiki/Skyrim:Riften_Jail... or worse." Mjoll: "They represent the reason I'm here. I can't just ignore them, Aerin." Aerin:
"I know. I just don't want you to leave; you're the only good thing that's happened to this city in a long time."
Since Maven and her family are essential (except for a couple members) I will have to settle with joining both the Dark Brotherhood and the Thieves Guild just so I can screw her family over as well as screw the Silver-Blood family over. If either of them try to touch me I would have the Dark Brotherhood pay them a visit, or they might have a werewolf knocking on their doors. My wolf only kills those who are not innocent or those who can defend themselves better than the ordinary citizens.
By the way that means the city guard is on my lunch menu and that also means the Blackbriars and the Silver-Bloods would be on it too.
Also which one is better? Killing Grelod the Kind just so you can destroy the Dark Brotherhood or killing Maven Blackbriar without having to kill any other citizens of Riften?
If you do not know who Grelod the Kind is then go to the Orphanage (in fact in Skyrim if you want to adopt you need to kill her XD) and open a few doors to see what she does. Ironically you don't even get a bounty for killing her because apparently nobody (including Mjoll) gives a [censored] about Grelod.