I have a few theories as to why the tape was in a locker instead of still in the tape recorder, as you'd expect from a guy who's just been eviscerated:
(1) Maybe the guy in his last few seconds saw it necessary to somehow pull himself over to the locker, open it, put the tape in and close the door, all the while being killed.
(2) Perhaps the baddy decided to recover the tape and stash it inside the locker for some reason.
(3) Perhaps... it was just an oversight by the developers.
Keep in mind, tapes are going to be used a lot more in the full game, so the devlopers needed a situation where there was a tape separate from a tape player, and the player had to learn the method for listening to tapes in the game. So I suppose they didn't want the tape to be inside the player already, even if it made more logical sense.