It's not left or right, it's authoritarianism mixed with a desire to expand and conquer, backed by a population who feel hard done by and willing to start marginalising members of that society, leading to human rights abuses that is the problem. You can get there from any side of the political spectrum. The state-heavy left directly takes control and you have corruption, nepotism and personality politics. The state-light right sells all for personal profit and simply manipulates through commercial and broadcasting means and private interests. Either route can become authoritarian, dupe their own people, invade countries under false pretences, or cover up abuses. Political stance isn't a moral indicator. At it's simplest, left or right is big state vs small state. You can get problems either way, or even from a mixed/ central position. If you have a corrupt government then they have all the tools they need to be corrupt in various ways, no matter the system, no matter the colour of rosette, and no matter their politics. That's the nature of authority and hierarchy. Who watches the watchmen? In a theoretical democracy the population do but of course there are ways around this through mass manipulation. Whatever check you put on power there will always be an element of trust as in a hierarchy the person at the top has all the tools they need, if they desire, to be corrupt and a degree of corruption is a guarantee when humans are involved.
Side note on Hitler. Many people on the right try to put him on the left. Many people on the left try to put him on the right. They are both missing the point I think. From my admittedly limited knowledge he doesn't seem like someone most interested in economic systems. I put him in the Genghis Khan category - an old style conqueror bent on claiming as much land, glory and power as possible for his country and to reflect back on him, rationalised through a bastardised interpretation of the Nietzche 'will to power' work, empowered through charisma, a tight rein on all around him, and historical circumstances. The political party or part of the spectrum they claim to be from is irrelevant as they are essentially throwbacks to the conquerors of old and dreams of world-spanning empires.
So socialism is one possible route to a political regime, willing to carry out genocide and human rights abuses and determined to claim land, power, wealth, and glory but it's not the only vehicle. It is just a system ultimately and presently it isn't even the prominent ideology in more than a handful of countries of the war-torn world(*) we live in.
(* It's actually probably not that war-torn relative to our history.)