"Terminate process" doesn't work for Windows 8 apps

That's an intended behavior.
First, when a Metro app process is killed, the taskbar button doesn't go away. You can try it and see.
Second, I'm not sure how Metro app processes are implemented. Perhaps it's possible that two apps share the same process, I don't know.
Why would you want such functionality? As far as I know, Metro apps don't tend to hand like native apps sometimes do, so it's usually enough to just close them.