Premise: Leftism is the putrefaction that arises from the decaying body of a once creative and correctly ordered civilisation that has abandoned or lost touch with its guiding principles.
Response: A less harsh appraisal might be to say that leftism is what arises when a society loses proper spiritual understanding and transfers the energies that pertain to the spiritual level to the material. So in its distorted way it is a response to spiritual impulse but on the wrong level and, as such, is anti-spiritual since it purloins what applies to a higher area of life and misapplies it to a lower, thereby effectively shutting out the higher. Its fundamental principle is egalitarianism and the end result of this, indeed its intention when you understand that it is put forward and encouraged by demonic powers, is to shut out access to the higher and reduce everything to the lower.
But why does all this come about? The answer is that leftism is at root a rebellion against God and the natural order of creation. We may dress it up and say it's a desire for fairness prompted by compassion which, of course, is how it would define itself, but much of that is window dressing and masks an underlying motive of spiritual rebellion. I believe research has shown that leftists are often more selfish and unfeeling in their personal behaviour once you take them out of the warm waters of ideological abstract theory.
I was thinking the other day of the English Civil War which was one of the earliest manifestations of what became leftism. The old joke was that the Roundheads (the leftists of their day) were right but repulsive and the Cavaliers were wrong but romantic. However, in a situation such as this right and wrong are not so easily assigned. There was moral right and wrong on both sides. Also, both sides would have believed, or believed they believed, in God. And yet despite the relative good and bad of each side's moral position in worldly terms there was a fundamental difference and one that brings out the division between the two sides today. One side saw this world in terms of the higher reality. Down here should conform to up there. King Charles' belief in the divine right of kings comes from this. We may not share this belief but we can recognise someone for whom all power flowed from the divine and who saw this world in terms of the next. The other side saw the higher world to a large extent in terms of this world. Its priorities, despite its strong religious convictions, were to do with justice in this world. I say they had strong religious convictions and they did but their descendants today do not for the most part and this is precisely because of their antecedents' priorities. They have inherited the worldly focus and as a result lost the religious sensibility because that was always secondary. Even though the Roundheads would have seen themselves as highly religious their religion had lost its proper supernatural grounding.
This pattern repeated itself in various ways throughout the ensuing battles between left and right but the right henceforth was never pure as such. It always absorbed more and more of the leftist ethos as it fought one losing battle after another. Every time it lost it regrouped, took on some of its opponent's ideology and the damage to spiritual focus became more severe. This is what I mean by leftism being a rebellion against God. Its ends are to further the human being's growth in a context voided of a true spiritual centre which would be in God. They will adopt a false spiritual centre to fit in with their self-serving ends but it is always secondary, never primary.
Of course, rebellion against God is not the only motivating force behind leftism. There are praiseworthy elements such as the desire to help the poor, the weak and other victims of society. It could never have taken root and spread so widely if that were not the case, and that is why it is difficult to criticise in a world that doesn't recognise God. But the good in it should not disguise its basic spiritual rottenness.
I haven't attempted a full definition of leftism here because its focus constantly changes as does the ground on which it situates itself. But basically any world view which does not have God at its centre is leftist and when I say God I mean the true God who has a spiritual purpose for human souls and who puts them on this Earth to fulfil that purpose. There are many false gods who come about to answer various human desires and deficiencies.
Conclusion: Leftism is a spiritual disease akin to atheism. It can only end in nihilism.
Cure: The reorientation of the human heart to spiritual reality.