Okay jack was appointed the leader in season one and did a good job,season two amongst all his head bobbing all he ever wanted to do was go fight the Others and sometimes just out of the blue,he would say "We need to form an army", then he would head out to go get them and stop and play cards with Sawyer.
So now in season 3, he is with the others,and is willing to leave all our Losties behind to cure Ben and get on a submarine and go home.
Now he comes back and wants to blow all the Others to hell.
Twice Sayid has to stop him once about the transmitter belonging to Naomi to get it to work another time Sayid has to remind Jack that the main goal is to get off the island.(Sayid shoudl be the leader). Jack is once again all gung ho warrior to blow the others all to hell,yet all he wanted to do before was get off the island himself.Now he is like who cares about getting off the island,let's fight.
But I think if Jack was given the option again to get off the island by himself he would take it. I don't trust Jack at all.