I'll take a stab at your question.

I consider Mussolini to be fundamentally different than some of the 'worst' guys like Hitler and Stalin. To those two I would add also, Pol Pot, Mao, and the Kim Family at large if we're making a list of the most truly awful world leaders.

This isn't to say that Mussolini was a great guy - he was absolutely a bad guy. Yet for all of that he was appointed to prime minister by a constitutional monarch, King Victor Emmanuel, and with the consent of his head of state proceeded to increase his power and militarize the Italian state. But again, it's important to stress that much of this was done with the consent of monarch and significant popular support. Then, once King Victor Emmanuel saw the tide turning he simply informed Mussolini that he was out of a job and had Il Duce arrested as he left the palace.

It's kind of a bizarre thing to think of a dictator being constrained by essentially what amounted to checks and balances on his executive power, but that was the case with Mussolini. Such an ouster of Hitler or Stalin would be unimaginable precisely because, unlike Mussolini, these men were modern day equivalents of absolute monarchs whose whims and prejudices totally drove the functions of the states they headed.

Moreover, Hitler and Stalin (who I'll focus more on, since they're contemporaries of Mussolini) were responsible for many millions of deaths between them in mass killings, deliberate starvation, and warfare. Hitler's fall from power entailed the ruination and division of Germany, and ended in the rubble of Berlin itself. Stalin's death left in its wake a system of overwhelming oppression and division in what had been a (for the most part) free Eastern Europe with effects arguably still felt today, to say nothing of the direct deaths and purges in his wake. Compared to these two - or for that matter, the 'Nation-sized Concentration Camp' of North Korea, or the massed misery of Mao's 'Great Leap Forward' - Mussolini seems relatively tame.

It's absolutely true that Mussolini eliminated elections (with the consent of the King). It's certainly true that he oppressed and suppressed certain elements of his population based on their political or religious affiliation yet at the same time he long refused to deport Italy's Jews to Hitler's death camps. The Italian military certainly committed war crimes with Mussolini's approval or at the very least acquiescence, but then under a different system of justice the fire bombing of Tokyo and Dresden, to say nothing of the Atomic bombs, could probably have been called war crimes. But I argue that none of these elements themselves are significant enough to warrant inclusion among the 'worst of the worst'.

Ultimately Mussolini should not, in my view, be seen as a uniquely evil or depraved individual. He was instead a politician adept at channeling a populist movement into nationalistic goals and who ultimately proved to be a failure as a warlord to ruin of himself and his movement. With a little different set of circumstances, it would be easy to remember him more as a Franco, or a Pinochet - a 'son of a bitch' that [The West] could do business with, whatever his personal failings.

Remember that Mussolini was first and foremost a 'Flamboyant Tough Guy' working for what he saw as the good and necessary expansion of his state - not an incurable racist bent on a 'Final Solution'. For a good part of the 1930's he was actually actively seeking ways to block Hitler's expansion and flirted with an alliance with the French (as well as the Hungarians and others) to that end. Had things turned out a little differently he might have ended up on the 'right side' of World War Two but as is he's usually mentioned in the same breath as 'Hitler' which adds a little villainy to his name.

Despicable person, cast in the mold of a particularly adept dictator? Sure. Worst of the worst, shoulder to shoulder with Stalin and Hitler? Absolutely not.

Sources:

Mussolini and his Generals by John Gooch

Jews in Italy under Fascist and Nazi rule, 1922-1945 by Joshua Zimmerman