Whether or not there's a 2x4 lying around in the house, there's a lot more serious issue here.
When I was seven years old, I understood the concept of hurting other people. I also had a clear understanding that hurting other people was WRONG.
The kid either has a serious chemical imbalance, or there are some serious issues in his upbringing. Both are possible --there have been some children who have been found to have some extreme EI (emotional impairment) pproblems that make it difficult to impossible for them to understand the consequences of their actions once they have lost their temper. A few years back I saw a kid in a situation where he felt cornered (got embarrassed about being asked to sit down and be quiet by a teacher) enough to nearly start throwing punches at a teacher in class.
At the same time, there's plenty of situations where parents no longer raise their kids --the television, XBox, DVD player, or school system now does. In those cases, there isn't the necessary infrastructure to show those children what right and wrong is (or that actions have consequences), and the results can be pretty frightening. In those cases, it's a sad thing that there is no preliminary requirement for becoming a parent, other than being legally old enough to procreate.
Either way, it sounds to me like this kid needs to be in an environment that allows for some serious therapy, or this will become a repetetive cycle of violence. I'd bet this isn't the first time that this child has acted out --just the most shocking.
Intel Q9450 @3.2GHz, Gigabyte GA EP45-UD3P, 4 x 2GB G.Skill @4-4-4-12
Antec P160SW case (modded), Xigmatek 750w PSU
3x 500GB (RAID-5), , OptiArc 7200S, ASUS E818A3T
Creative X-Fi XtremeGamer, Hauppauge HVR-1800, Radeon 4890