Why is creating undead Evil?

[Xgya]

Magic that creates a whole new body and can otherwise bring people back from complete disintegration cannot bring the deceased back to life until their undead forms, even mindless ones, are once again put to rest.

Someone whose body parts were used to create a flesh golem can be resurrected by the Resurrection spell if you have an unused part of their dead bodies available.
If it was raised as a lowly zombie, the mighty True Resurrection cannot do a thing.

I'd say this is because undeath corrupts or captures the soul somehow.
That being the case, stealing someone's afterlife is an evil act that deprives them of their own liberty.

Even divine intervention from a god of life and death (using such as the Life and Death salient divine ability) is unclear as to whether it can resurrect someone who still walks the earth as a skeleton. It states it can resurrect someone whose soul is trapped, but otherwise works like True Resurrection, that works on people that got turned undead and then got destroyed, but not otherwise.

As for the 'still the same creature' argument (which I like a lot for sure) then that just means undeath is a form of slavery. You're forcing someone to do a thing they didn't (or can't) agree to, and that interpretation makes it clear it's the same person.
Also, unlike golems, whose purpose for creation was to mindlessly serve, Timmy the Barber, now Timmy the zombie, clearly was not.
Unless undead creation creates a whole new creature, in which case we turn back to my soul corruption argument above :)