Leftism is materialism, and materialism has it that human life ends with biological death and has no meaning. Thus a our life has no other goal than hedonism - to be got-through as pleasantly, and with as little suffering as possible.
Yet Leftism include some of the most fanatical, zealous, venomous, ruthless moralists ever to have lived.
There is an apparent contradiction. But since this contradiction is seen in so many tens of millions of people; in private as well as in almost all mainstream, high status, public discourse - it can't really be a contradiction.
How can moral relativists also be moralists? How can solipsistically-selfish hedonists also be maniacal humanitarians, pathological altruists and suicidal empaths?
Well, this apparent-contradiction is statistically-normal in the modern West, it is mandatory - anything else is severely sanctioned; but what is going-on?
Given that there is zero philosophical coherence, I must assume that there is a strong psychological coherence - and it must be quite a simple and straightforward kind of coherence, since so many people seem to be able to hold Leftist views, over the long term, and resistant to alternatives.
Fortunately, I have the advantage of having been a Leftist atheist for most of my life - so all I need to do is look back on what was my own state of mind. And that yields the simple answer that my psychological coherence came from what I was against, not what I was 'for'.
As a Leftist atheist, I knew what I was Not - and I knew this very clearly. I was not a Christian, I did not believe in God or anything spiritual. Just like tens of millions of others - I was against all forms of authority justified by tradition. I was against distinctions of class, sex and race. I was against the infliction of suffering. I was against Conservatives, Republicans, and any other party that was identified as to the Right.
This is psychologically coherent, and it is perfectly simple on the basis of 'us' and 'them', in-group and out-group. Throughout human history; most people, most of the time have defined themselves more strongly by what they hate than by what they love.
(While it seems that everybody can hate; not all that many people seem able to love...)
Indeed - what people are 'for' is very seldom coherent; and people don't seem to mind all that much - probably because people think in short, detached, sequences.
When incoherence is noticed, this requires holding several things in mind simultaneously (in 'working memory'), which is difficult to manage and impossible to sustain... and then this fades-away and the incoherence problem disappears.
So we can see that having major motivations defined negatively and by opposition is quite 'normal', even when it makes no sense.
On the other hand, it does make a difference.
In the short term - well, there no problem from incoherence. After all we can only do one thing a a time. People simply focus on the task in hand...
But holding mutually incompatible convictions over the long-term, across a span of human life - well, then, and inevitably, contradiction makes that life meaningless, lacking in purpose, and at war with itself.
And this is what we find.