so what did onigumo actually try to achieve?
I can only see the mc's resentment increasing towards the marines and the world government by making him come here and that just seems counterproductive to me
Have the MC brute force his explanation afterwards by using his sun/flame powers to cover the slaves' hoof marks with a bigger scar/burn and then saying to any future interrogators that he never saw any hoof mark.
MC is going to be an advocate of his own brand of Justice! Equal Justice! Everyone will be punished for harming the innocent, be they Civilians, Pirate, Marine or even the Nobles! muhahahah!!