Racist, homophobic and misogynistic tweets that Milwaukee Brewers reliever Josh Hader sent in 2011 and 2012 surfaced as he pitched in Tuesday night’s All-Star Game at Nationals Park, turning his appearance into an embarrassing stain for Hader and a public-relations nightmare for Major League Baseball. After Hader surrendered a three-run homer in the eighth inning, several Twitter users — starting, it seems, with an account named MLB Insider Dinger — found and retweeted messages Hader sent as a 17-year-old. The tweets included numerous uses of the n-word and an allusion to “white power” next to an emoji of a closed...