Does one have to be evil to be a killer? Can they be pushed over the line by the environment they are forced into. Jack has slowly become desensitized to violence. It is normal to a degree. When put in a survival situation, most people will defend themselves and those they care about. But, once you get on that path....... can you get back off of it? Even Sawyer has become desensitized.... a Killer. He killed Tom much easier than the other two men he had killed. Sometimes I think the point of the show is not so much good vs. evil.... as the fact that all of us have the capacity for both given the right circumstance.
I like Jack, I think he is a good and honorable person. I just think he got off track and needs to find his way back.
Many of the survivors are killers.
Ben: Killed Dad, and orchestrated Dharma slaughter.
Kate: killed father, possibly caused ex boyfriends death.
Hurley: feels he killed the people on the deck, and his curse caused other deaths. He directly killed the guy he ran over at camp.
Sayid.....Iraqui soldier torturer
Eko..... Brother, Drug dealers etc.

Anyway, you see what my point is I could keep going awhile...lol.
Our losties are all pretty capable at killing.
It is a theme.... and yes I think Jack is too.