The solutions above are fine, but you'll lose the chance to get an unique and powerfull item: the ghoul mask. Wear it, and you'll be safe from all feral ghouls that you'll meet on your path.
To get it follow these steps
- get the mission from Gustavo to kill Roy (and get the weapon)
- go find Roy (pay attention to feral ghouls and traps on your way down the metro)
- speak with Roy and propose to live in peace with humans at Tenpenny Tower
- go back to Tenpenny and ask him to let the ghouls get in the Tower
- Tenpenny will give you some caps if you convince tenants Edgar Wellington II, Millicent Wellington, Anthony Ling, Lydia Montenegro and Susan Lancaster to have the ghouls as neighbors
- Convince them to leave the Tower (speech check) or kill them all but with karma loss or read below the easiest way.
- Return to Tenpenny (if you plain to kill him for the "Gotta Shoot 'Em in the Head" mission, now is the moment: you've just this small window to do it)
- Now come back to Roy and tell him that ghouls are allowed to move to the tower.
- Get the prize (the mask and 300 XP reward)
Easiest way to convince the 5 people:
Take the Love Letter from Susan Lancaster's desk and show it to Millicent Wellington, who will then proceed to murder both her husband Edgar and his lover Susan. Millicent will then leave the tower, leaving only Anthony and Lydia. These two can be convinced with easy speech checks or stealing from their store safes and speaking to them after doing so.
If you come back a few later you'll find humans and ghouls live toghether in peace, but a few more day after, you can find that all humans are gone. Ghouls, infact, have killed them all: theirs bodies can be found in the storage room. Find Roy and ask him about humans... now you can kill all the ghouls without getting bad karma.