
Jehan Alain was tragically and heroically killed in the second world war, and his music was primarily popularized by his sister, Marie-Claire Alain, whom I consider to be the one who understood his pieces and played them the best. I also think she's one of the best performers of Bach I've ever heard. He was a young man who was a very talented organist and composer, and who really liked modernist (of the time) music, but who came from a pious and musical Catholic family, and who died too young. I would listen to Marie-Claire Alain's renditions of his work.
A litany is a repetitive petition, and like a lot of expressionists of his day, I think Jehan was trying to show what is going on "behind the scenes," that is the power and energy of the prayer that isn't necessarily tangible. I think the chromaticism is more meant to express glory or intensity than it is "madness." It's not that easy to play those chromatic passages that fast! But both Jehan and his sister were extremely talented organists. I feel like if they're played too slow, you might not get the right impression! If you read the Wikipedia article, it quotes the preface he wrote for it, which helps to understand his vision of the piece.