The theory about a long held grudge could be possible, but why now? Could it be Jayme changing schools? The person only found out where they live because Jayme was in the Barron school now?
Or the person moved away for a while and only recently moved back?
Maybe it is random and they thought no one was home because they did shoot near the house or go around peeking in windows and thought the occupants weren't home. In that case they wouldn't be planning to kill anyone so the shock of their plan going so bad could have been why they left so quickly.