First of all, heiter lived a hella long life, around 104 years.
Second of all, if you want to look at a long-living creature trying to understand humanity I suggest also to read "to you, the immortal"
The fact he found the grimoire in a TOMB should have told her enough about the resurrection and immortality magic inside it: either it didn't work or the price was too high...
At first I thought he became afraid of death because he grew selfish. I was wrong, it was the opposite.
He fears death because he can't see himself leaving someone dear to him helpless. Something he didn't consider in his youth.
He's pretty cunning tho, tricking Frieren into spending years training his adopted daughter eventho she didn't want to at first. What a nice old priest.
he lives a very long life, especially for a chronic drunkard. and I'd say it's pretty fulfilling too.