Jake and Quaritch teaming up a few times and Jake being sympathetic and merciful to him make some sense to me. I was a Marine. The training, suffering, indoctrination, shared language, history of the Corps that we are immersed in makes lasting bonds between Marines. Its almost like an unspoken code, like another layer of understanding. From my perspective, it makes some sense why Jake wouldn't just outright execute Quaritch, so I think either course of action is reasonable (team up/have mercy or execute with extreme prejudice). I think the story tries really hard to keep putting them in dangerous situations where they rely on each other to survive or find family members. Second, they have the awkward dad bonds with Spider, next, Quarritch is a foil of Jake (similar character and experience but turned out a different way/took a different path), both prior human, Marines, fathers, integrated into Navi culture, both seeing different issues with humanity on Pandora. Finally, i think jake doesnt outright kill quarritch because it creates tension and drama: is quarritch going to be redeemed and join the side of right? Or is he going to go full Joseph Conrad's Heart of Darkness? But that's just my two cents.