Prion diseases, specifically Transmissible Spongiform Encephalitis (TSE).
Prions are a protein that, as near as we can tell, all mammals produce. These proteins are not broken down during digestion, so contaminated food is a popular vector.
Creatures typically create their own variants of the Prion Protein (PrP), so Prion diseases are typically not zoonotic (they do not cross species barriers). However, cattle do have a zoonotic form with humans, in Bovine Spongiform Encephalitis (BSE, Mad Cow Disease).
It takes humans nearly 20 years to be symptomatic, but once symptoms begin, progression is rapid (months) and always fatal. Prion diseases are neurodegenerative disorders, which will cause behavioral changes, rapid cognitive decline, and intense pain.
Prion diseases work by an animal taking in a malstructured PrP that, through some mechanism that isn't definitively identified yet, slowly changes normal PrPs to the malstructured form. Since normal PrPs are necessary for these creatures to live, the immune system does not respond to the malformed PrP.
The most common form of transmission of Prion diseases is cannibalism. Since humans don't generally eat each other, this is incredibly rare. However, improperly buried corpses can contaminate water supplies and crops. Additionally, PrPs are very resilient in the open air -- An intentional delivery system would be to just spray it directly on crops.
The malformed PrPs can appear spontaneously in animals, so people don't need to be directly exposed in order to get Prion diseases, but spontaneous infection is rare, and there is no known mechanism to intentionally create a spontaneously malformed PrP. Plants have been known to take up animal PrPs, so you can't simply wash your fruits and vegetables, as the infectious part is already inside your food.
The only plausible (though unlikely) way to infect the entire world would be orbital chemical weapons caches that accidentally deorbit, spreading large quantities of malformed PrPs throughout the entire atmosphere and eventually depositing them on every field -- or some other similar global contamination mechanism. It would take a decade before the first people become symptomatic.
Animal-based vectors are out... i.e., you wouldn't be able to weaponize mosquitos to carry it from person to person the way malaria is carried, because humans exist in all biomes. The only other animal that shares all biomes with humans are rats, and since humans no longer have fleas like we did back in the days when rats carried the plague throughout europe, a weaponized Prion disease wouldn't be transmissible that way, either.
Global TSE infection would have the most gentle impact on the environment of any human die off event. There won't be planes crashing, etc., as symptoms appear slowly and are not immediately debilitating. Power plants and dangerous reactors will be shut down in controlled manners. Residual pollution will slowly leak into water supplies as storage containers break down, but this is a decades-long process rather than an immediate event.
Pets and farm animals will be released and hunted by natural predators in a more drawn out manner... Pigs will be the most successful invasive species.
As far as the die off itself... Some people will start getting more violent as the pain and cognitive decline drives them insane. There won't be roving bands of violent half rotten zombies; they'll just take their frustration out on the people closest to them, suddenly start getting very weak (their brains won't be sending strong signals to their muscles), then over the course of a couple days, lay in bed (or in an alley, or wherever) and eventually stop breathing. The die off will take a very long time in human timescales, but all humans will be dead within 25 years of releasing the weaponized PrP into the upper atmosphere.