Subway gang Leader: You look like a sympathetic dude. My partner here needs an operation, but we're a little short on cash if you know what I mean.
King Willie: There's no stopping what can't be stopped, no killing what can't be killed.
Lieutenant Mike Harrigan: You're rippin' my dick off and shovin' it up my asshole.
El Scorpio: Come and get it! El Scorpio is ready.
Peter Keyes: It's taken us over two weeks to learn his patterns. He comes here every two days to feed. Seems he has a taste for beef.
Lieutenant Mike Harrigan: I didn't think he was a vegetarian.
Peter Keyes: Well, we've prepared a trap for this predator.
Lieutenant Mike Harrigan: Captain, Danny and I came up together. Fifteen years on the fucking street! Whoever killed him's gonna pay. I'm gonna finish it.
Answer: The predators are honour bound hunters, and as such they will only attack a target if it is capable of defending itself (i.e, if it is armed, which is why it doesn't attack the boy in the cemetary). Although the police outside were armed as well, the Predator presumably attacked the drug dealers because they were a harder target (as the police were outside he could have just sniped them from the roof tops, but he had to get relatively close inside the building) and as such, killing them gave a higher honour. The obsession with Danny Glover's character is probably because he is quite a brave and heroic person, similar to a Predator (the way he saves the other cop by risking himself etc.), so the predator may have been studying him for a while. There's also more honour in killing someone like that (as he'd be a hard target) than there is in just killing a random armed guy.
Gary O'Reilly