How do leftists justify the 130 million + people who have died at the hands of socialism?

Don't have to justify anything.

That is the penalties of failing to adopt Communist identity.

Stalin, Paranoia.

Mao, the 5 year plans of urbanites over agriculture.

Hitler, Industrial elimination of the Worlds Desire util the later parts of the war, the whole world was against Jews, (The Pope said, they killed Christ.) Russian had its pogroms, The Turks had their "night of the long knives", the USA had its Democrat's KKK. Even after the war there was the "Gentleman's Agreements".

Put shortly, they can't.  But they certainly wouldn't mind upping the count for the chance to try it again.   They just don't realize that useful idiots get the bullet too.

The mentioned leaders were not true socialists.  Hitler was a national socialist, Mao a communist, Stalin a communist.  

Being a LEFTIST means never having to say 'I'm sorry.' Notice that, life for the survivors of Lenin, Stalin, Hitler, and Mao was no bed of roses either. Waiting for the 'knock on the door' for a FREE trip 'to the death camps' in the REST of a now miserable life,

They think the crocodile they're riding will kindly eat them last.  

Sure, every socialist is a communist dictator, btw, Hitler was a right wing nationalist.

If you meet your history teacher again, slap him for doing such a bad job

They weren't killed by socialism.  They were killed by megalomaniacal authoritarians, like Donald Trump.

You didn’t send back your stimulus check did you? I want what the people want.

You will hear nothing. Liberals NEVER SPEAK THE TRUTH