So, before I joined The Dark Brotherhood and played their questline, I was quite startled about how............evil the guys were.

Now, don't get me wrong, I think the DB's questline is one of the best in the game, but I was wondering if it was good or bad in a game if a joinable faction was inherently good or evil in an RPG ?

For my part, it's a little silly to pretend a cadre of assasins should be good, the business of killing is a seriously inmoral trade, and it's certainly going to be filled with rather unpleasent people, same for the Thieves Guild, which also incurr in some rather ugly activities, their very first quest pretty much tells you that to join the faction you HAVE to get your hands dirty.

However, I think it becomes a little difficult to role-play if you are forced to do things that fall outside the moral boundaries of the character you've created, should the game offer viable options to avoid going out of character or should it stick to a particular moral "palette" ?

So what do you guys think ? Should guilds lack morality or be distinguishably good or evil ?

The dark brotherhood's pretty evil. If you're roleplaying a good character you should probably just not join. There's even a quest you can accept to destroy it.

Id say that guilds should have a good or evil vibe to them but still should be morally 'grey' so that any RPG character can play their questline and still feel authentic. For instance the Dark Brotherhood shouldve had some sort of redeeming quality or character with such a quality that would make them mostly evil but believable like,say, they used to be a mostly moral guild of legal assassins (see:morag tong-elder scrolls morrowind) but some leader or enemy ended up corrupting them or forcing them to change their ways. The companions kinda had that in their inner circle being werewolves which everyone hates vampires and werewolves in Tamriel.

As it stands, the Dark Brotherhood had a great questline, but they were 80's cartoonish in their 'evilness'

Elder Scrolls IV: Oblivion Dark Brotherhood is waaay more hardcore.

That locked house party scenario, for example, was super intense!

Are there any inherently good factions in Skyrim?

I thought the Thieves Guild had a good bit of Morality tie into their storyline. I think that was my favorite of the guilds in Skyrimjob.