How did a disgraced airman, convicted by court-martial of assaulting his wife and child, get access to the weapons he used Nov. 5 to commit the deadliest mass shooting in an American house of worship? The short answer seems to be: not legally. But the case of Devin Patrick Kelley, who murdered 26 parishioners before dying of a gunshot in his flipped car, illuminates a confusing tangle of federal regulations that are supposed to keep guns out of domestic abusers’ hands.