The subject line says it all. He was arrested for gun-related crimes a couple times but not charged and certainly not convicted of anything. While he was in the Navy, he was disciplined eight times but managed to keep his "Secret" security clearance.. In civilian life, he gave signs of having anger issues and loved to play violent video games.
In effect, despite all that, he had fairly spotless record.
All this raises the issue of how effective background checks could be.