You really should do the questline, it's the best in the game.

And the targets, they are easy to kill. They are all eminantly killable and/or at the least, very annoying. Apart from the last.
Apart from the last, none of them are totally innocent.

Apart from the last one, the game is less annoying with them dead.

To join the DB or to wipe it out, that is the question.

The Oblivion Quest line was very good, and some of actually defined what it was to be an assassin, a master of death, a dealer in being able to kill efficiently.

In Skyrim, You run around the countryside killing off Bandits, what wrong have they done that can be defined? Some of them are Gambling usurers, some of them are sitting off the road and taking down Caravans. You kill them without honing your skill of killing Bandits. You may even be told to go out and take down Forsworn. They are rebels in a land that once was theirs, and have some right to a "Self Rule situation" Most times, you do not even think of their struggle as you plough through them.

In Oblivion, the DB Guild was a Gulid of Professional Killers, to be hired to kill, with often very proscribed methods. There was one where you were meant to look like you had killed. That one was very well constructed.

In Morrowind, if you held the Contract, the Powers that Were respected the Contract (but that was the Morag tong). You had respect as a Professional Killer.

Here in Skyrim, it is not so much the Professional Guild that the DB used to be. Some of the Quest line is very gray of shade. But someone asked for a Professional Killer to be sent, has paid the Due to Sithis, the Night Mother and to the Guild. At least there are real reasons for the Killings to occur.

You can chose to be a Professional or not. The game requires you typically to kill to proceed. Sometimes IMHO, it is good to display the Skills you develop and try to Rebuild what once was a very Honoured and Professional Guild.