They were not just thieves of a very powerful artifact that was not theirs, they were the killers of a king's wife who spent centuries and millennia together, whose love for each other managed to survive in legend.
Furthermore, we have to take into consideration who is meting out the punishment here, you can make an argument that sealing until the end of the world is an unjust punishment if it were enacted by humans, as they cannot fully grasp this concept with their lives brief, but a god? especially a death god? He fully understands the penalty and its consequences. This argument fails because you only source the human perspective, not the divine perspective.
Furthermore, I would like to mention that Lydia is only willing to give them a chance to talk to them because she was not affected by their actions, not knowing her father's former wife or what he was like before. But I assure you if they had done this to Daniel under the same circumstances (him jumping in front of her to protect her and being stabbed with a sword) she would have had as much mercy for the killers as her father had or less.