If death wasn't a thing what would be the most unforgivable crime?

Burying someone alive. Or trapping their feet in cement shoes and throwing them into the ocean.

This question implies death is the worst that could happen to you.
Personally, I don't see it that way. There's a lot more hurtful things than death.
Death is one occurrence, with no pain in the death itself, and "only" secondary pain in those left behind.
There's a lot more hurtful and lasting pain you can inflict, physically and psychologically, and without a definite endpoint.

Not putting your shopping carts back.

Kidnapping, I say, in a world where killing a death is simply not a concept, being kidnapped for any reason could lead to millennia of torment

This one gets more complicated the longer I think about it.
My first pass was to imagine humans just as we are aside from the ability to die. Many things about how humans are don't make sense without death though. Pain, for example likely evolved to cause organisms to avoid stimuli that could lead to their death. Fear largely derives from the anticipation of pain. Would true immortals have either? I imagine the psychology of such creatures would be vastly different from our own.
There's also the question of what form the immortality takes. If it's possible to destroy someone's physical body, but their soul can immediately manifest a new one, and pain doesn't exist, then doing so is just an inconvenience. If bodies are impervious to any damage or alteration, a large category of crimes vanishes.
It would probably come down to some sort of long-term imposition on the freedom of others, but it's really hard to guess what that would look like.

It'll still be rape.
- God, imagine the person who downvoted this
- Yeah, almost put it in the question too, but felt that would be rude
  
  Yeah that was probably a good call, even I hovered for a second before posting, but rape has always felt worse in my book, like killing someone is a horrible thing to do to someone, but they don't have to deal with it for life, the other one is a life long thing that could be argued is worse than death.

Any sex related crime.
- Why?
  Edit: why would it be worse than when death is a thing? And why would it the "most" unforgivable?
  
  Becauae that shit stays with you... and if you can't die, you have to live with it for an eternity.

Assuming that immortality only applies to humans, environmental destruction would be a big one.
People care more about pollution and climate change when they know they'll be around to face the consequences.

Unauthorised pregnancy. If no one can die, every new birth is effectively stealing from the limited pool of resources. Too many births, everyone starves, no one can die to ease the burden on the limited resources. Endless suffering for all.
- Abortion would probably be impossible too
  
  They could shoot any unauthorized births a random direction out into space. Not like they care where they end up
  
  I was gonna make a joke about ending the abortion debate, but then I went down a rabbit hole:
  Abortions procedures would still be a thing, but the fetus wouldnt die. Its just an extremely premature birth. That means that intentional pregnancies could be "harvested" early, for the fetus to grow up outside the mothers body.
  Lots to unpack.
- Wait, what resources would be most valuable if death was off the table?
  Edit: I'll take a stab at it and say cats.
  
  Seriously (or at least as serious as a hypothetical invinciblity/immortality scenario can be :D): food water and housing. You can live without most other things, but being cold, wet, starving and dehydrated will really make the eternity drag on :)
  
  UNLIMITED CATS 😼
- Its no death. Not no aging or sickness.
  People are still going to fall apart as they age. So I wonder if there would be a hard cap where you basically have a farewell party and go into a euthanasia booth/coffin.
  Unauthorized pregnancies would be criminal. No idea what that world would do with the child in that situation.
  
  Euthanasia is still death though, so unless live people are just going to crawl into a box and willingly never come out? And if they do get out, are they going to be okay with having being starved in complete isolation?
  And where are their bodies stored? Outside of few exceptions, most human bodies decay away within a hundred years, so the grave sites can be reused, but you cant do that if someone's still in it?
  If the world allows voluntary death, then it gets a bit easier to deal with though.
  
  Separate comment, because I think it deserves its own discussion, but can there really be aging and illness? It might get a bit philosophical, but if all your cells stop regenerating and die off, eventually you'll end up a bit of ooze that can't interpret or perceive the world, so at that point you would be dead?
  In my interpretation of the scenario, it would be like "in time", everyone grows up to 18 (or some arbitrary age), and then you are stuck like that forever after?
  How are you envisaging it?
- Well some might end up in the mines and others as the first proper space explorers who are just yeeted into space. Deep sea exploration without a submarine might also be on the table.
  
  Its much harder to enslave immortal beings. How do you force someone to work in the mines?

Stealing from musk.

Unforgiveable implies that there can be no mistake about the negative side effects for even the dumbest among us before committing the act. Torture is about as intentionally cruel as it gets.
Some other suggestions here are frequently used under the umbrella of torture, for example, by the IDF.

Permanent mutilation

If death weren't a thing, I suppose that that pitching someone into lava or something like that would be pretty bad then.
- Only if you're a pussy and can't take a few million years trapped in the earth's crust.

No death? As in you can't die from old age or you can't die no matter what happens to you e.g you're throw into the sun, you don't die? Or your physical body can die but that just means your game is over in one universe and you can move into the next, and so on?
If being thrown into the sun can't kill you, then you're invulnerable and torture can't be a thing. You could be tossed into deep space and not hit anything for a million years, but you could learn how to cope after a few years and make your brain a retreat of imagination.
If it's just games all the way up into eternity, being the game creator and making pain exist is an unforgiveable crime.
- If being thrown into the sun can't kill you, then you're invulnerable and torture can't be a thing.
  I disagree. there several forms of torture that don't involve killing you
  
  How do you hurt somebody who is invulnerable and doesn't need to breathe? How do you torture somebody who can survive being crushed by the sun? Tickles?

Throwing someone into a universe where nothing exists...
Enjoy an eternity of absolutely nothing (and you can't even die 🫠

Torture. The main thing about torture is to make sure they don't die while suffering the pain.
- I cant imagine what would be the most heinous torture in a world like this. Maybe casually, "accidentally", stepping on the same person's toe ever other day for like one or two million years.

Burying people alive

Torture, kidnapping.

In my opinion killing is pretty bad but there are other crimes that can be worse, so not sure what point you are trying to make
- Death doesn't mean killing. You can die by an accident (falling off a ladder). You can die from a heart attack. Death just means dying.
  
  Then even more to my point. What difference would it make?

slavery?