Bob is made a very obvious sacrificial lamb, being Alice's best friend/boyfriend, getting lots of screen time, etc. Only Charlie, who is white and has stayed in the background, is killed first instead.
...except Charlie is actually Not Quite Dead, and while gazing in horror at his "corpse" Bob is really killed first, possibly by Charlie, who was faking his death to lure Bob into a trap.
Later on, Bob dies for real. He's the first casualty in the story, in fact.
Bob is incredibly unlucky, comically falling victim to Everything Trying to Kill You. Maybe drawn out and subverted several times over for maximum comic effect, but ultimately something does get Bob. Or, Bob is a black They Killed Kenny Again character, with much lampshading.
An arrow about to hit a Mighty Whitey changes its course at the last second to hit Bob.
Same as double subverted, but Bob knew Charlie was trying to kill him and either fakes his own death or else avoids Charlie's attack, thus allowing Bob to kill or apprehend him as so desired.
Races don't determine who dies first.
The cast is mixed race, so the deaths of various ethnicities are spread out.
Bob either survives, or dies at some other point in the story without his ethnicity having any significance or relevance to this
OR: Bob lives long enough to be the only living hero or, at least, dies last.
The entire cast is black, or there are no black cast members in the first place
In order to woo young black viewers, the studio insists that Bob, as a black superstar, be cast. But Bob is very busy and can only be on set for a couple of hours. The script is hastily rewritten to ensure Bob dies first, thereby allowing his name to be plastered all over the production without risking Bob's other commitments.
The director is a racist. So are his fans. Killing off black people is expected of him, it's one of his trademarks. Of course, the actor who plays Bob didn't know this.......
The characters weren't originally meant to be a specific race or ethnicity. But the casting agency decided this black guy would be perfect for the role of Bob who is the first to die.
"Hey, Bob, how come Alice died when you were standing right next to her?"
"Okay guys, we're a bunch of teens and a serial killer is on the loose. We all know that serial killers go for the black dude first (you can beat the crap out of me later, Bob) or for defenseless young girls (same for you, Alice). Therefore, all us white males should do whatever we can to protect the black guy and the girl while we all get the hell out of here. As long as Alice and Bob are alive, then the rest of us are pretty much invincible (hopefully)."
Discussed: "Don't you hate it how the black guy always dies first in this sort of situation?".
Conversed: "I like horror films, but it's a bit jarring how the black guy always dies first".
Deconstructed: Bob spends all of his time cowering in fear due to the existance of this trope.
Reconstructed: Bob receives constant death threats from racist groups, perhaps due to his being outspoken about an issue which negatively impacts upon them. Therefore, he spends all his time cowering in fear from the potential invokation of the trope. Ultimately, near the start of the story, the threats are acted upon.
Intended Audience Reaction: The writers made Bob the first guy to die in the story to set the audience up for a Plot Twist: When Bob died, he went to hell. But since he was a Badass, he managed to fight his way out. However, he only actually comes back to life near the end, when the rest of the gang face the murderer. More than half of the fandom is shocked, and the twist is so well-known that it becomes an It Was His Sled situation (forgetting the rest of the film, by the way).
Played For Drama: The highly racist Caucasian cast conspire to ensure that Bob is always in the most danger, thus making it inevitable that he'll die first. Alice is appalled by this, and a major theme of the rest of the story is the conflict between those like her who tried and failed to protect Bob, and those who ensured he was the first victim. The drama is even greater if this scenario, takes place before the Emancipation, thus bringing overtones of Fair for Its Day into the mix.