Audio problem: Garcia is typing on a flat, foldable rubber keyboard but the keys' sound is loud like from a plastic keyboard. The rubber keyboard shouldn't make noise. (00:39:00)
Other mistake: A mugshot of a suspect is showing with some personal information. Name, license number, address, etc. The field for "Home address" shows a valid address but "Work Address" shows "Male." The correct field title should be "Sex." (00:40:00)
Other mistake: There is a mugshot of a 35 year old suspect on Garcia's computer. According to what she is saying, the suspect's mother died 10 years ago. A photo shows her with the suspect and he looks exactly the same age as he does now. (00:40:00)
Continuity mistake: After Emily shoots the unsub in the head and he falls on the bed, the gun falls beside him. A second later they show her walking over to his body to remove the gun from his hand. (00:51:00)
Answer: "The Boogeyman", s02e06. Spoiler alert: The kid is Jeffrey Charles and at that point he had already killed 3 other kids. Jefferey basically hated and resented other kids after his mother abandoned him and his father spent a lot of time with other children because of his job. Jeffery would lure kids Into the Woods and then beat them to death with his bat just because of hate, no particular "reason." The scene you're describing, Jefferey agrees to walk Tracy home when her "buddy" leaves her (at this point the FBI announced there's a serial killer on the loose and kids should walk with a buddy). Jeffrey gets mad at Tracy for walking too slow and for complaining, and he takes a swing at her, causing her to flee Into the Woods and have Jeffery chase her.
Bishop73