Mitchell Erickson’s fingers began dialing 911 last week before he had a chance to even consider alternatives, when two black teenagers who looked to be 15, at most, cornered him outside his home a block away from the park. One of the boys pointed a gun at Mr. Erickson’s chest, demanding his car keys. Flustered, Mr. Erickson handed over a set, but it turned out to be house keys. The teenagers got frustrated and ran off, then stole a different car down the street. Mr. Erickson said later that he would not cooperate with prosecutors in a case against the...