Well let's see, the thing is there is no law per say, but if you got bad karma you will get bounty hunters after you, same thing if you're good for some reason, don't know who want me dead for helping people... "Raahr him helping some wastelanders is filling me with murderous raaaage!" to make a reference to ZP.
However you can't for example kill someone in a town without everyone turning against you, if for example, person A is holding a key to person A's place, where Item B is and you want it, you can't kill him without having to kill everyone in the town, and the wasteland isn't exactly overpopulated so you need places where to trade and so on^^ unless you have the mister sandman perk but then you can't get to person A because he's behind locked doors during sleep, you could steel but I seem to get caught even with many skill points on sneak...
And don't nuke Megaton before taking the Streangt bobblehead! Hope it's been helpful!