You do that in exchange of killing Moira Brown and her Mercenary.
I DO NOT CLAIM THIS: SOURCE WIKIPEDIA:
"It is possible to obtain a Rock-It Launcher very early in the game. Upon exiting Vault 101 proceed to Megaton and enter the shop: Craterside Supply. Inside you will find Moira Brown and a mercenary. On the workbench nearby you will find a bottlecap mine. Grab, not take, the bottlecap mine and go into sneak mode. Make yourself hidden, then take the bottlecap mine (You will lose karma). Then place the bottlecap mine in between Moira Brown and the mercenary. If they are not close enough to the mine it won't detonate. Once they are dead loot Moira Brown's body for her key; use it to open her supply locker. Take all of the required components and the schematics for the Rock-It Launcher. Then activate the workbench and build the Rock-It Launcher. "
That will kill them stealthy. Also Moira's quest rewards can svck my duck .
Here is the video anyway. I made it 1080p. Enjoy http://www.youtube.com/watch?v=zq5qba6d3Xo

That's pretty sweet :foodndrink: never really thought about it. However, i tend to keep Moira alive (due to laziness, not becase of any of the moral or ethical crap) as she's quite a reliable merchant smack in the middle of things. Will keep this in mind for any evil big-guns char I start off in the future!

I loved how Moira acted crazy and detonated herself XD. Also it won't alert any of the "villagers" in Megaton. To be 100% sure just wait 24 hours. As for merchanting she sells only junk like tin cans. Also you can donate to the church to boost off your karma if it went to bad.