Can't Kill Kids in Fallout 3?

I have the PS3 version of FO3 and I'm playing through it with bad karma to get all the trophies, When i first got to megaton i killed lucas simms (the sheriff) and now when i walk around (i have the sheriff hat and coat on) the megaton settlers are like "Hey whats up sheriff" but lucas simms' kid is all depressed and annoying so i tried to blow his head off he just runs and hides and you can't do V.A.T.S. either, but when you go to shoot at him the arrow thing turns red. if i cant kill the it's fine i was just wondering if anyone else had. and has anyone else killed lucas simms? haha