grimdank

It seems like you've mostly read about Drukhari from second hand sources as opposed to the novels or codexes, or at least from recent ones.

The Drukhari LITERALLY get off on being evil.

It is more like eating a good meal than anything else based on descriptions in multiple novels.

They know they’re doing wrong,

Define 'wrong', they are from a different culture and morality is always veiwed through a cultural lense. There is no universal morality and wasps are proof of that.

That it keeps Slaanesh at bay temporarily in pure self interest

You eat living things because you must (yes plants included) out of pure self interest. Again morality is not universal, if you believe it is please show me a kilo of compassion or a gram of altruism.

the Aeldar, who they could join btw

Potentially join the Craftworlders, as many would fail and die before they could effectively mask themselves from Slaanesh. Going from Craftworlder to Corsair to Drukhari is much easier than the reverse.

And for the record the Craftworlders all worship a god of murder... which they have a lot of issues with and do recognise as a moral problem. Not necessarily an improvement.

Plus it ignores that the Ynnari succeeded and offer a viable alternative and every single Drukhari that didn’t immediately convert

The overwhelming majority don't know for certain that the Ynnari's claims are true, and they're from a society where trust is nearly non-existant. Read the Queen of Blades novel about Lelith Hexperax for some good insight on this point. Again Ynnead is not exactly a morally good god, and is at best neutral, as it is a god of death.