With the military dead, his master murdered, and the other personnel fleeing for the copter, Bub is left on his own with nothing but zombies for company. A sad fate that leaves one wondering if he will gradually gain in intelligence while all those around him remain flesh-hungry and he himself gradually rots, or if he'll just degenerate into a mindless automaton.
The opening scene after the Jump Scare. The team searching for other survivors calling out in vain for them only to be met by nothing, but more Ghouls and the local wildlife. It's an extra sadness upon the realization that there may be no one left alive on earth anymore.