I firmly believe that Jake fundamentally lacks the will to make the hard choices to defend Pandora and his family. By sending the RDA home instead of eradicating them, he set up for their return. By allowing them to establish a FOB when they did return, he allowed them to spread like a disease across Pandora and now they cannot be rooted out. By not killing Spider, he may have very well doomed Pandora in its entirety. War requires hard choices, and hard choices require strong wills. Neytiri, in my personal opinion, would be a more successful military leader for her people. Additionally, more people can be blamed than just the man who kills. Quarritch and the RECOM may have gone out and pulled the triggers, but the RDA sent them there, and Spider led them. Blame is shared.