So this kid has been described by his peers as having been the target of some pretty severe bullying throughout his childhood. I'm sure he also saw how the former, and likely future President relentlessly directed similar behavior towards many people. Not only that, but he surely witnessed many people both on social media and in his daily life joining in on this behavior, and celebrating it. It's really not too hard to see how a person like Crooks could have been motivated to do what he did.