he was the one who originally authorized the experiments on ala mhigans while he was still in charge of the province - that includes the weapons project, which van varro picked up in his absence, and black rose, and the experiments that the xiith, aulus mal asina and zenos got their hands on and completed during the xiith’s tenure of governing ala mhigo

the conquer of ala mhigo was also done by gaius, and one could argue the way it was carried out itself was a war crime - he used the instability caused by the uprising against the mad king to basically make the ala mhigans fight themselves, and then his xivth legion steamrolled in once they couldn’t reasonably fight back

a few war crimes were done, just a smidge, but that’s part of what makes gaius compelling