What is more potent than inhaling nitrous oxide and die of exhaustion from laughing of course? I'm trying to wipe out humanity using our own nervous system against us but hope someone can provide me with a clue on how I can litter the world with countless decomposing clowns. I think something colorless odorless and tasteless and maybe airborne to cover large area quickly and I want to end the world as quickly as 1 year is it reasonable can I hope to have the last laugh?
Well, increasing the Oxygen content to much higher levels could make everyone happy and feel good. Get it up high enough and then strike a match and watch the world burn...
Humans are adaptable and intelligent, so even if you can kill off %50 in a relatively short period of time, people would find ways to avoid the deadly gas. So I think it unlikely, some kind of super germ might be more likely, especially one that causes a euphoria like some of the illegal party drugs. Something that is highly contagious and hard to kill, or at least no one wants to be cured.
Find a remote way to trigger the pleasure centres of the brain and leave it stuck to permanent "on". Rats with electrodes implanted in their brain's pleasure centres will continue to push the pedal that triggers the electrode even if they are physically starving to death, so this is not really science fiction. (In science fiction, Larry Niven described a society where some people did this to themselves; they were referred to as "wire heads". An alien race also discovered how to remotely trigger the pleasure centres of the human brain as a way of controlling humans co-opted for dangerous missions).
This could be done on a global basis if the signal can be sent via cell phone towers and wi-fi routers, which would give virtually global coverage (seriously, even in the depths of South Sudan, cattle herders keep track of things like market prices using smart phones). The signal transmitters would have to be different to interact with human brains, but the placing of transmitters could be explained away as "system upgrades to enable 5G connectivity".
Once the signal is sent, human activity would cease as the vast majority of people become paralyzed by euphoria. Taking the signal away would crash people into depression and lethargy, wanting desperately to recreate the feeling. If the signal is left running continually, people will starve to death in the streets, but totally happy.