Revealing mistake: When Will Smith throws a rock at the escaping Edgar alien, if you watch the rock after it hits the alien, it disappears after hitting the ground. (01:23:20)
Revealing mistake: When Beatrice turns the sugar bowl over to dump the sugar into the glass, a barcode label and handwritten prop inventory number with the name 'Sony' can be seen on the bottom of the bowl. The film was made at Sony Pictures Studios. (00:15:15)
Revealing mistake: There is a scene where Agent J is standing on a car and shoots the "Noisy Cricket" at the fleeing Edgar. The camera shot is from inside the car and just before Agent J crashes through the windshield, you see a precut area in the glass. (01:01:25)
Revealing mistake: When J and K get into the car (a POS if memory serves) K makes a comment about J fastening his seatbelt, if you watch in the background out of J's window as they pull away there are a few people walking at an incredible rate showing that they filmed the scene at a normal speed then sped it up when put into the final cut. (00:40:40)
Revealing mistake: When the alien takes off Edgar's skin, when he throws it to the top of the hole in the ground then drags it back, you can see its rubber skin. I read the correction too. And no. Human skin doesn't look like rubber glove material.
Revealing mistake: When K and J buy the "hotsheets" (newspapers) to look for information, the clerk who K gives the money to briefly looks at the camera. (00:44:00)
Revealing mistake: In the scene after J shoots the exterminator van off of the tow truck he shoots again. When the camera clips to the tow truck driving away, you can see the smoke generators emitting smoke from the curb. (01:03:50)
Revealing mistake: As K walks out from the car, saying "INS, Division Six", look at the road they just drove up. It runs a short distance, and ends at a hill of dirt. Presumably, the pile and the end of the road are at the sound stage wall.
Revealing mistake: When J and K are knocked over by the roach near the end, when they land you can see a square of grass move. Shows that a crash mat is covered with a bit of grass to make the landing easier. (01:19:20)
Revealing mistake: When the jeweler's ear pops-open, the angle changes and it gets replaced for a prop that looks nothing like the real ear. (00:54:35)
Revealing mistake: When Arquillian is laying flat dead in the morgue, his pulse on his throat is very visible. (00:50:35)
Revealing mistake: When James Edwards (later, J) catches the alien in the beginning and has him next to a wall, the alien pulls out a "reverberating carbonizer with mutate capacity" to shoot J. A quick tussle, and the alien drops the gun. When the camera goes to a close up on the gun, watch carefully. You see it quickly fade out of view after it comes to rest, and replaced with a few burning sparkle effects to simulate disintegration.
Answer: K and his partner both knew about aliens living on Earth, and made it their duty to make sure no one else knew about it. When the trooper saw Mikey in his real form, and Mikey spotted him, he attacked the trooper in an attempt to kill him, probably to ensure that the trooper wouldn't be alive to expose them. Since K and the other agent were already aware of Mikey, as they had dealings with him before, Mikey didn't attack them.