AACHEN, Germany — The shooter in last week’s attack on a synagogue in eastern Germany filmed his rampage and uploaded the live feed to Twitch, a video game-streaming platform. Rather than explain why the man had escaped the authorities’ notice, however, the country’s interior minister, Horst Seehofer, suggested monitoring the country’s video gamers.

This prompted an outraged response from one of the most trenchant and widely followed critics of Germany’s political establishment, a 27-year-old, blue-haired YouTuber known only by his nom de plume, Rezo. “How can you repeatedly screw up your job so badly?” he asked of Mr. Seehofer, adding: “He and his crew are so unbelievably incompetent.”

His response over his Twitter account — which is celebrated by his legions of fans and carefully watched by the traditional news media — included language not fit to print and a warning: “Enlighten your parents and grandparents so that no one votes for this party.”

It was not the first time that Rezo had lit into Chancellor Angela Merkel’s government. Five months ago, he burst into the German mainstream with a 55-minute carefully written YouTube polemic telling his followers not to vote for Merkel’s conservative party in the elections for the European Parliament.