>Some neckbeard on Reddit thinks the Imperium of Man is somehow the worst faction
The only path of thought I can think of to justify this statement is the belief that the Imperium cannot wait to become the next galaxy spanning dictatorship.
But yes the Eldar are (and have been) always capable of unspeakable horrors. Just swing by Commorragh any day of the week without a proper disguise. Part of the issue is that folks fail to understand what the webway and Commorragh truly are, and this is not their fault. This is lack of adept writing and general disinterest in the topic, in favor of publishing more books about what Dante was wearing when he met with Lion, or whatever the hell Asclepius Latin-name is going to do to the baddies with his fellow ultramarines company.
Reminder they could all use spirit stones to cut themselves off from Slaanesh entirely like the craftworlders do but they seem to refuse that for some reason.
Craftworld spirit stones don't work like that. They are a backup plan for when your ass is KIA and you are far away from the craftworld infinity circuit (your soul depository in general). It will hold on to you, preventing Slaanesh from eating your soul straight out, but if those stones break, or someone less scrupulous finds them, you'll be in a world of hurt. A psychic-atuned soul is needed, which is why the commorrites don't have them as they have atrophied their senses for over ten thousands years of sado-hedonism.
The exodites can escape by linking their waystones to a world-network so when they die they literally join with the planet's essence.
Harlequins are different in which they ritualistically give their soul towards the care of the Laughing god. No one outside of the Black Library knows whether this works or not.
Outcasts and corsairs truly live on their own terms, sometimes completely forsaking the stones which gives them the risk-high they seek (and is often unavailable in the safety of a craftworld)
There's a final, even more haphazard and mysterious way of subverting Slaanesh. Should a craftworlder give into his or her own desire for battle, bloodlust beyond reason and outright carnage, they will "trap" themselves on the path of Khaine and become aspect shrine exarchs. The end result being that those souls join a gestalt of previous exarchs, currently coexisting as an amalgalm of souls within the multiple stones embedded on the exarch armor. Not unlike a that jap tale of possessed samurai armor and bloodthirsty sword.