Lest we forget the very systems enabling this behavior of banal evil were created and maintained by perhaps the most evil of them all.

The word "lest" has a meaning that does not make sense in the way you've used it.

The word "lest" is meant to come after something else. It means "for fear that" or "or else" or something similar. So it's not clear what you're trying to say. I don't know what "very systems" built by "the most evil" you mean either. Is it supposed to be obvious what you're referring to?

It's from a poem in 1897.

https://en.wikipedia.org/wiki/Recessional_(poem)

Oh, okay.

I clicked the link and remain confused. The poem does have in common the term "lest we forget", but apparently nothing else. It's also used in a grammatically correct way in the poem, but not in your post, since it's not following anything.