Revealing mistake: At the very start of the film, when the car blows up, if you look to the right you can see a piece of debris (it looks like a car door) flying through the air. Look on either side of it and you can see the black cable it's gliding on. (00:02:45)
Revealing mistake: The police car that is shot at and flips over doesn't have an engine. (00:02:50)
Revealing mistake: When the drug lord falls from the roof he lands on a table yet it doesn't collapse under his weight. The building was pretty tall. (00:10:25)
Revealing mistake: When Danny falls down from the ceiling, watch closely and you can see the security wire attached to his leg. (00:34:40)
Revealing mistake: When the subway train slams on the brakes, the camera cuts to a shot showing the sparks coming off the wheels as the train is slowing down. If you look at the wheels carefully, you can see that the train is already stationery, and the camera is panning down the train to give the impression its slowing down. (00:58:30)
Revealing mistake: When the predator fires his plasma caster in the slaughterhouse scene, you can see that the simple laser blast has been replaced by a Chinese sparkling firework on a string. In the original "Predator" you could see this same effect on a string but it was not nearly as spectacular and easier to miss. (01:17:50)
Revealing mistake: At the slaughterhouse fight, both the Predator and Harrigan shoot simultaneously, Harrigan with the machine gun's grenade launcher and Predator with the gun on his left arm. When the shot is fired, visible on the Predator's weapon is a wire/fuse. (01:17:50)
Revealing mistake: In the slaughterhouse scene, look carefully after the predator throws the disc to Keyes. The disc slices the beef that are hanging, but just before it slices Keyes, there is a split-second stop. (01:21:50)
Revealing mistake: When Glover slices off the Predator's hand with the disc, you can see the hand has already been cut and then re-attached to the costume (there's a very visible dividing line between the two). (01:25:30)
Revealing mistake: In the scene where the Predator has fallen into the bathroom of an old couples apartment, you see all types of building materials such as brick, wood, and such laying on the floor from the impact. If you look closely, some of the bricks actually bounce up and down after the Predator hits the floor. (01:25:50)
Revealing mistake: When the Predator runs out of the bathroom after doing minor surgery on himself, you can see that when he punches through the bathroom wall and into the living room and from the living room into the hallway, that both sets of doors had been pre-cut to achieve this effect. Even with the amount of force applied by the Predator, to do this considering how powerful it is, the wooden slats that were used in the old days to give homes extra strength and to help plaster adhere to walls would certainly have been more rugged looking than they were as in the movie they are neatly cut into the shape that the Predator, had to go through. (01:29:55)
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