Magnum Opus - S8-E13
Continuity mistake: In the scene where you first watch the killer drain a victim's blood, the upper part of the glass jar is coated with blood, but when he puts it in the fridge this part of the jar is clear. On the other hand some of the jars in the fridge are still fully coated with blood, which shouldn't be as the blood had the time to gather at the bottom. (00:07:15)
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