Sounds like "Nature's End" by <PERSON> & <PERSON>
* Explosive population growth
+Ironically (as I type the actual world population is quoted as 7.12 Billion according to The US Census Bureau) this is set at "over 7 Billion" but the "Depopulationist Manifesto" is quoted as saying "The reason that life on earth is in danger of destruction is that there is an overpopulation of human beings".
* Widespread global pollution
+The first chapter has the son "<PERSON>" of the main POV character "<PERSON>" dying trying to save others "in the Denver pollution catastrophe of 2021".
* one of the antagonists is a cult leader who preaches to people to voluntarily commit suicide to reduce burden on society
+This character is <PERSON>" the head of the "Depopulationist Movement". "Every person who joins the Depopulation will gain much from his death and rise far in his next life."
* There is some backdrop story to this antagonist in which he is revealed to be a sadist torturing little animals
+"Once I got a cat and tied it by its tail under the window, and cut a small slit in its belly. 322
It screamed and squirmed about, and once in a while I would stick my fingers inside -"
* The protagonist races against time to find a solution for the pollution which is becoming lethal
+This is the only thing that doesn't compute. They are racing to complete a "conviction" - a computer simulacrum of a personality which can be questioned and will always tell the truth about the person it is simulating.
* he uses notes from his late son who was a researcher in the field to follow a trail. The research notes are in a data pad that self erases if certain questions, devised by his son, are not answered when reading it
+This follows the description of <PERSON>'s datafiles exactly, although I can't find the quote.
* the solution he finds at the end is a secret lab that has been growing super-intelligent next generation kids who have the potential to solve all the problems.
+It's not a lab, but a secret community of hyper-intelligent children and uplifted apes call "Magic".
Incidentally, this is one of my favourite books! :) 322