Morag Tong is a guild of religious zealots used as tools for dirty business of the Great Houses or murdering "undesirables" if rich, powerful people have such desire. I wouldn't call them "good", although it's true that they are a legal guild.
Hence the quotations around the phrase "good". While assassin's are rarely ever percieved as good people, they are often percieved as good when they're hired by important folk to kill those they brand as "evil", or "Heretics".
There needs to be some "good" assassin's includeed in the next game, but, the point is, the Dark Brotherhood are more like Mercenaries than anything.
They kill to appease the night mother.
They had a familiy sense for while, untill Lucien made you kill everyone. Talk about love you to death.
I know they aren't angry murders just Sociopaths.
Again, this is just Oblivion's portrayal, which really annoys me. They've made the Dark Brotherhood just another group of cliché bad guys. I mean, they were sent to kill the Nerevarine by King Helseth, and NOT by the "Night Mother". Sure, Bethesda, make some cliché angry murderer guild, but don't ruin existing guilds for it.
This isn't just the Dark Brotherhood, either.
The Mage's Guild was supposed to be a group of beurocrats with the occasional dedicated scholar. Not a typical group of good guys wanting to kill the necromancers. in Morrowind, for example, certain quests where quite questionable for a guild dediated to magic". Collecting 1,000 gold from some mage, by whatever means neccessary. For "Guild Dues". Quite questionable.
The Fighter's Guild was supposed to be a group of mercenaries. They have no morals. They'll do anything as long as it's legal and they earn a profit.
The Thieves Guild, I must admit, was done quite well, but, like Morrowind, they missed out all the extortion, bribery, muggings and general nastiness you would expect from a group of thieves.
I always thought that Blackwood company, who weren't even playable in Oblivion, were one of the more in depth guilds of the game.