While there are tons of chances to be evil in this game, it doesn't feel like there is an over-arching personality that let's you role play as an evil character. While I welcomed the simplistic moral compass of the karma system and the more morally ambiguous factions, it at least created a uniform idea of evil and good for the player to choose how they wanted to play. At best, you can be chaotic neutral. This raider option could address that. Imagine if when you first arrive in Concord you had the option to side with the Raiders and massacre the settlers and minuteman holed up inside? This could be used to trigger the raider main quest line (while failing the minuteman quest line). An option to be horrifically evil so early in the game isn't crazy: Fallout 3 has nuking Megaton and New Vegas has takking over Goodsprings with the Powdergangers, so it wouldn't be too far-fetched to have the option so early on to display a good amount of devilish behavior.
It would also allow for more interesting (and maybe peaceful) interactions with the different raider settlements in Fallout 4, like the Combat Zone, East City Downs, FMS Northern Star, Federal Ration Stockpile, and Libertalia.
I would gladly pay for a DLC where I gained the option to take over the Commonwealth... with FORCE!