The technology available in the Cosmic Era rather makes the Geneva convention a bit of an antique, plus new technologies present the option for greater crimes against humanity.
1. Viral Weaponry
Contagion, The Stand, Resident Evil, etc. Creating indiscriminate lethal micro-organisms is just a bad idea.
2. Slave Clones
creating sub-human races is just inviting lower caste rebellions, and all sorts of chaos and violence
It is genetically possible to create super humans, which is generally a bad idea for the rest of baseline humanity
4. Genetic Coercion
with tailoring an option it is possible to make people who have to eat certain GMOs and pills, otherwise they die.
No one wants to destroy the world in a puddle of grey goo
6. Orbital Bombardment
We promise to not shoot the planet with giant space guns
7. Mobile WMDs
We promise to not arm tanks and mecha with ABC weapons
8. Arcanotech WMDs
This is cutting edge tech that no one really understands so lets not even start on arcanotech superweapons
Terminator, The Matrix, etc. The Robots have an established record of defeating fleshy humans
Any other ideas???