One possibility would be create a pathogen that produces amphetamines or amphetamine like responses to the brain. Ampetamines, by flooding the brain with dopamines, has all kinds of wacky effects.
- Induced mania: Basically, the opposite of depression. Afflicted believe they are better looking, high social status, morally superior and physically superhuman ("Of course I can outshoot a SWAT team and get away.) Eventually, they believe themselves incapable of error and will react violently to anyone that disagrees with them, does not obey or other wise frustrates them anyway.
- Impairment induced paranoia: Paranoia is the brain's fail-safe mode which it falls back on if impaired. That why every thing from senility to way to much THC from otherwise largely innocuous marijuana, trigger paranoid behavior. Amphetamines bring this on big time both directly and through sleep deprivation.
- Sleep deprivation: Causes active hallucinations
- Hyperactivity: Afflicted cannot remain inactive.
Amphetamines always burn out because they cause dopamine release and when all the available dopamine is depleted, the crash occurs and the behaviors stop. But a pathogen could in principle, cause the dopamine producing neurons to go into overdrive constantly pumping it out.
The afflicted would be hyperactive, hallucinating, paranoid, feeling invulnerable, and justified in all their actions to the point they could morally attack anyone for the least real or imagined offense or disagreement.
They probably wouldn't riot in mass per se but they would start attacking people and infrastructure e.g. setting fires, poisoning water etc pretty much at random.
Remember, the lethality of any communicable disease is dependent on the lag time between infection and manifestation of systems. A disease that manifest in hours or days won't spread far before the vectors die or get quarantined. A disease that takes years to manifest can spread world wide, infect a large percentage of the population, before anyone even knows it exist.