Bill was just bound to get caught...

I'm not sure if it was Bill's original intention of getting away with the rampage or not, but he certainly didn't stand a chance of actually just sliding on by, unsuspected by law enforcement. Bill's only friend Evan was thought to have been the shooter, according to the news report at the end. If investigators thought Evan was really the killer, the first person they would've tracked down, would be his good friend...Bill. Friends are always a person of interest for investigators. They would've tracked down Bill, on the day, the minute they linked him as being Evan's friend. And plus, when Evan's dad began screaming "My son is innocent! Bill Williamson did it!", the police would've showed up at Bill's house in a heartbeat. And on top of that, you're telling me that there wasn't one security camera in that town, that could identify the shooter?? Unless Bill had an escape plan to begin with, I'd say he highly underestimated law enforcement. And even if he did have an escape plan, the last place he should've returned to was his parent's house. Killing Evan and trying to make him look like the shooter, actually hurt his chances of going undetected. Of course investigators are gonna want to have a talk with Bill. It was rather arrogant and almost unbelievable for Bill to just be chillin in his room, counting money, and pretending like there wasn't going to be any attention from law enforcement directed towards him. Being Evan's friend, he is definitely a person of interest. If Bill had any common sense, he should've split town, the minute the rampage had come to an end.