http://www.haskell.org/ghc/docs/latest/html/base/GHC.Conc.html says:
The killThread function may be defined in terms of throwTo:
killThread = throwTo (AsyncException ThreadKilled)
I think it should be:
killThread = flip throwTo (AsyncException ThreadKilled)
or, perhaps better:
killThread tid = throwTo tid (AsyncException ThreadKilled)