One of the brothers charged in the August shooting death of mother-of-four Nykea Aldridge in Chicago, which prompted a controversial tweet from Donald Trump appealing for black support (“VOTE TRUMP!”), was released only two weeks earlier on a firearms violation.

Ditto the murderer of 15-year-old Hadiya Pendleton, whose 2013 killing was adopted as a symbol by the Obama administration. Her killer had also recently been released on a weapons charge.

Chicago...