WiFiKill exists for root users to do one thing: knock someone off the Internet connection you're sharing with them, thus freeing up the bandwidth they were taking. It scans the subnet you're on for other devices, and once one is selected you can effectively knock any one you find off the network for as long as you're running WiFiKill.
While there are more effective ways to deal with bandwidth hogs, like talking to them (sometimes subtlety is required, such as when someone pulls up to within range and starts abusing an unsecure hotspot), it's still handy. It's also useful to get the kids offline and down to dinner as you can bump them off from the kitchen; no need to even yell "dinner's ready" or "shut down the XBox before I shut your face!"
The app lets you name any connected user or device it finds, which easily allows you to cut off people individually (e.g., sister's plugged-in stepson, buddy who thinks downloading torrents at your house keeps you both safe legally), without knocking valid devices off the net.
WiFiKill should be completely legal if you own the Internet connection you're using it on, or are authorized by the owner to use it. It might not be if you're on a public utility Internet connection such as at a public park or library. You'll need to consult a lawyer if you're going to go IP punting.
WiFiKill lasts until you shut the application or phone off, but you should treat it like everything along the network will need to be rebooted (i.e., don't try this in a hospital, at work, etc.).[Ponury.net]