i agree that jack is definitely changing due to the stress of being on the island.
in the case of the marshall, that was also euthanasia (sp?) because he was in pain and going to die a slow death anyway. but i too am surprised jack did not check naomi's body. i think that allowed for the cliffhanger of her disappearing, because had jack checked her then he would know she was still alive. but like i said, the stress, is definitely showing cracks in the character of our dear doc.

i still love him tho!!!