theory that J&B bottle has gasoline and NOT scotch
at the end.. McReady offers Childs a sip of "scotch" and Childs drinks it without hesitation.
there is a theory that the content was gasoline to make the Molotov cocktails used minutes earlier. in which case McReady KNOWS the thing is impersonating Childs as the alien creature can't tell the different liquids apart.
(McReady is seen about to drink it as Childs approaches.. in order to make the charade more believable)
hmmmmmmm. whatcha thing about that ?!