Dystopian. Man isolated underground for an experiment run by a science group.

[Danny]

When the guy emerges a large part of humanity has died off.
Returns to parents house just in time to save starving pet dog. Some of the story is told from the dog's perspective.
The man begins meeting up w/ other survivors and forms a community.
He meets up with a cult leader and drugged up group.
Cult leader believes he is messiah and kills a couple of people in the first group.
Cult leader gets his come upenses.

Thanks

[Kris]

Selection Event by Wayne Wightman seems to match your description.

Amazon's preview:
https://www.amazon.com/Selection-Even...

Google search of Goodreads:
site:goodreads.com dystopian man underground emerges experiment "dog" "cult"