Everything comes down to first principles. If you have the foundations of your view of the world wrong then whatever you build on top of that is going to be wrong too. Leftism is essentially an ideology that derives from a this-worldly focus. It is grounded in materialism and atheism (clearly, since the two rose together), even if many of its adherents might think of themselves as spiritual believers. But their spirituality is humanistic which means it puts man first and any idea of God is only fitted into the needs and aspirations of man as he is on this Earth. Basically, leftism is the religion of humanity and it denies God, if not literally then as the chief end to which human attention should be directed. This is why leftism is, when viewed in the light of a God-centred understanding, actually evil. That might seem an over-the-top thing to say but hear me out. Because leftism reduces good to material good ( which we may tritely sum up as being nice), it obscures the real good which is rooted in the transcendent and its values of Goodness, Beauty and Truth as conceived spiritually, and focused on the spiritual needs and proper destiny of the soul. Such an approach which diverts attention from spirit to matter, prioritising the latter over the former, does effectively make it an enemy of the true good, and that is why it is encouraged by those supernatural forces who wish to separate man from God which also means separate man from his own true self.
Although this anti-good aspect has always been present, it has only recently become clear that leftism ends up (because nothing stays the same and seeds sprout) by giving preference to quantity over quality and replacing a natural, hierarchical order with an imposed spiritual disorder that is liable to conclude in totalitarian control because that's the only way the disorder engendered by egalitarianism can be managed. The (undeniable) fact that the old order was corrupt was used as an excuse to sweep it away entirely instead of returning it to its proper origins in God. But the inroads made by leftism could only have been made in a society that had already begun to lose touch with spiritual reality since once the basis of order with its roots in God is no longer acknowledged then the whole edifice becomes shaky and all the component parts of it can be challenged. So there are valid grounds for saying that there was a real need for the goods that a leftist ethos represents to be introduced into society. But what was suitable or even necessary as a temporary medicine became harmful when transformed into a permanent diet.
If all this had been instigated by love, love of the poor, the left behind, the suffering, then the results would not have been so bad. And, of course, there is this aspect to it. There was real concern for those groups and for people who were seen as the world's underdogs. There was a real need to redress an unfair balance in society. But this was not all there was to it and, increasingly, it became less and less of a factor. The real motivating forces behind much of the rise of the left over the last couple of centuries were resentment and hatred, and this has been easily exploited by the demons who pervert the course of history for their own anti spiritual ends.
Essentially, and despite its good points which, of course, it has or it would not have been the success it has been, leftism is a rebellion against God. If you don't see that it may be because you are not properly orientated in your heart. You might have a spiritual health problem as so many people in the world do today. The fact that leftism, though false, derives from the misconception of a truth (the oneness of humanity) might make it harder to see, but a rightly ordered person should still be able to see that the truth has been distorted and twisted out of its proper context. Some people do see the falseness of the left and how it will result in a death spiral but their reaction to it takes place on the same level and is therefore part of the same materialistic/secular process. What is required is a higher response that shifts the whole debate to a spiritual level. Then leftists not solely motivated by grievance would see that the things they value are included but their relevance will be considered in a new light. For the greater includes the lesser but if you try to force the greater into the framework of the lesser, you mutilate it.
Leftism is an ideology. It is not based on an understanding of the human being seen in the overall perspective of what it really is but on a limited (material) part of it that has no independent reality. In contrast, traditions may need to be modified as humanity evolves (and it does, human consciousness does not remain the same) but they are built on real experience over centuries. They do not come from theories or abstractions but from life itself. And when they are modified, they are modified not uprooted and thrown out. The growth should be organic like that of a tree not artificial like a machine. Christ did not come to abolish the law or the prophets but to fulfil them.
Some of the ideas behind the left arose in response to a real need at one time. There is still a need for them. But this should be within the overall context of the world as traditionally conceived; that is to say, as spiritually conceived. It should not replace that but supplement it, as and where needed, and it should always be secondary to that. Now, unfortunately, it has supplanted it and what may once have been nourishing has become a poison. There is no doubt that leftist ideas arose at a time when humanity was discovering itself anew, and they were part of that discovery which, roughly defined, involved the advance into a more creative consciousness, more focused on the individual. But when they became separated from higher spiritual understanding and taken on their own terms the good they might have done turned to harm. The servant usurped the role of the master, and when natural hierarchies are toppled the only outcome possible is disorder and chaos.