Okay but why did diabel from the first season refuse the /health potion??/ Kirito was trying to use on him before he died? I don’t see the point on stopping him if it could have prevented his literal death

Potions in SAO take some time to take effect, so Diabel’s health wouldn’t actually be restored if he had drank it; Kirito would just be wasting a valuable survival resource on someone who was beyond saving.

However, recovery potions in this game were really frustrating items, having only a «Heal Over Time» effect…in other words, drinking the bottle would not instantly recover the gauge, but instead make it gradually increase in small increments […] - Sword Art Online Progressive 1: Chapter 1, Part 14

That said, Kirito offering Diabel a healing potion and cradling him in his arms is an anime-only thing; in the original novel Diabel dies almost instantly.