Mussolini was not elected. The legitimate majority government was taht of Luigi Facta’s Liberal Party.
Mussolini led a march on Rome on October 27th. King Victor Emmanuel III refused to support Facta’s call for military authority to put down the march, fearing civil war. King Victor Emmanuel III asked Mussolini to form a new government on October 29th.
True he was not in the majority party. But the fascists had been elected to parliament. Mussolini’s march on Rome was a manipulation of parliamentary rules by Mussolini. It was gamble, but under tthe rules in place at the time his ascent to power was legitimate. It’s what the left always does, bend, distort and manipulate the rules.