Is there a draw back from killing these jerks

I always do it, kill them all and then go in and loot the place. Go all Boondock Saints on their asses.

There is no drawback in killing them. They only sell you a small amount of energy weapon ammunition at a time and all the energy weapon mods they sell can be found at gun runners.

are you interested in completing the quest Birds Of A Feather? Are you interested in completing the quest Heartache By The Number for the non-violent outcome? Are you a frequent user of Energy Weapons?

If you answered Yes to any of these then you may not want to kill them, or atleast not until you've completed one of the two quests mentioned above as they are mutually exclusive

Otherwise, have at it!