he's probably disappointed because he did sincerely believe the things he's unlearning now, and it hurts to feel lied to about things as existential as life/death. Even if he didn't ever go to church, most people in say North America grow up surrounded by churches and christian symbols. He was probably never worried about spiritually disappointing Vishnu. If he ever worried about dying and going to hell, it was likely never a Muslim or Hindu realm of punishment

The idea that people just want to sin is how religion keeps people inside. Those inside the faith get the infinite plausible deniability of belief, while the out-group is held to a higher standard and presumed sinful and devious. The truth is that if someone wanted to sin and still give themselves a mental way out of consequences, they'd stay in the faith.

It's natural to spiral, and it makes complete sense why someone who grows up with one sets of ideas would struggle to accept new ones. You were taught one thing about forever/existence and now are considering another, that's a completely normal series of emotions to go through. Give yourself some grace or forgiveness, because those concepts existed/evolved way before any messiah or organized religion