Exit Wounds - S5-E21
Factual error: The BAU is called in to assist the Sheriff's Dept. in Franklin, Alaska (a made-up town, although there is a ghost town by that name). By statute, Alaska has never had any Sheriffs. Functions filled by sheriffs elsewhere fall mostly under the auspices of the Alaska State Troopers. Other law enforcement is by community or (rarely) borough which is what Alaska has instead of counties.
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