Monkey D. Luffy wroteI'm a High Elf who's specialty is Fire... But I don't know whether I should join or destroy because of my light affiliations
What would you advise me to do???
Answering this question is a bit tough since i could throw some spoilers, however you can't enter the Dark Brotherhood sanctuary without being affiliated with them, this is mainly because there is a password that needs to be said. Thus my question, how could you destroy them? Now, what you could do is start their quest line until you are given the password, and then kill everyone inside, you pretty much will get all the good gear out of their bodies, but might miss out on shadowmere horse. However, that's not a big issue since there is another cool horse called Frost which you can get somewhere else, or so i read.
The Dark brotherhood aren't really "evil" sort to speak, people, that need someone killed, do a ritual called black sacriment, this is where the night mother will send one of her assassins to kill someone. Usually those that are killed did something wrong at some point in time. Therefore, they have an enemy.
If you still lost on what to do, just Ignore them till the end, then decide.
If you are curious on what happens in the Dark Brotherhood Quest line, read spoiler:
*****SPOILER******
Just like Oblivion, you will be betrayed at the end which leads to destroying the sanctuary, you will then move to another sanctuary and buy new furniture making it a new home for you if you haven't got one already. You get a new shout-like skill that calls an assassin to fight next to you, and you can have an assassin companion follow you. You will keep getting quests directly from the night mother to kill someone, someplace, and earn 800 gold per kill. Of course, not to mention getting shadowmere halfway the quest line.