It's hard to reconstruct a joke based on its last two lines.
I'd guess that perhaps two people John and Bill are in a dangerous situation, e.g. walking through the jungle, and one suddenly disappears. According to the comedian, John, if white, would ask, "where's Billy, and walk around wondering about 'where's Billy?' and calling for him. John, if black, would simply realize it's a VERY dangerous situation, Billy's probably dead from an unknown danger *AND this 'thing', most likely, is about to strike again,* and start running to save his (J's) ass.
The joke is about white people's inclination to discuss things, discuss, wonder about in a clueless way, as opposed to black people's (alleged) figuring out the obvious and taking the obvious remedial action, esp. when one's life is on the line.
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There is a routine by Steve Harvey at youtube, called
Flash Back Friday :: Getting Fired
which has a similar theme: white people as unrealistic.
ADDED: I think the general point behind the humor, is that from a Black person's point of view, the white person has the luxury, in many situations, of being in no hurry, having time to reflect (even talk to one's analyst!). The Black person needs to (and does) figure out a situation in a hurry, and take action. If a white person is shopping in a store, and sees the store detective frown while looking at him, he might think, "I wonder if he's frowning at me.... Well, maybe he is, but it could be because he thinks I'm about to steal something OR because he's angry at something unrelated. Maybe I remind him of an old enemy. Hmmm. Let's weigh the evidence." The Black guy, will think, "I'm suspected of being a thief; I better keep my hands in sight, leave any items for purchase behind and get the hell out of here--without appearing unduly in a hurry-- before I'm detained and accused." And he acts immediately and accordingly.
