Fun facts, according to Quran in human very first murder crime when one of Adam's sons killed his younger brother out of disagreement on offerings to God, the crow showed him how to properly bury the dead body:
Then Allah sent a crow digging a grave in the ground for a dead crow, in order to show him how to bury the corpse of his brother. He cried, “Alas! Have I even failed to be like this crow and bury the corpse of my brother?” So he became regretful.
Do you have to have seen a human die to know that a human could die? Surely Cain had seen plenty of animals die. I think you might not be giving Cain enough credit (or maybe too much credit) by assuming he couldn’t have put 2 and 2 together
Then Allah sent a crow digging a grave in the ground for a dead crow, in order to show him how to bury the corpse of his brother. He cried, “Alas! Have I even failed to be like this crow and bury the corpse of my brother?” So he became regretful.
https://quran.com/en/al-maidah/31