In the surveillance video, 11-year-old William McQuain glides along the pavement, wearing sneakers with wheels on them. Then comes another image, one revealed in court Thursday: A man furtively carries a baseball bat and slips it into the back of an SUV. The two ride off. A short time later, according to the timeline prosecutors laid out, the man slammed the bat into William’s head and hid his body in a wooded area of Montgomery County.