Is Jack solely guilty of how the story ends in the savagery?
Jack is not the only one responsible for the novel ending in the savagery that it does. Everyone except Simon and Piggy are responsible for the way things turn out. True, Roger and Jack are the main antagonists, but everyone else let the savagery happen and even participated in it too. How does the famous phrase go? All that is necessary for evil men to triumph is for good men to do nothing.
No. Jack is the leader of the band of savages, but what if none of them participated in the savagery? What if they all refused to hunt and kill? This is comparable to Nazi Germany. Not all of the citizens agreed with what Hitler was doing, but they followed along, most of them out of fear and/or the need to feel like part of a group.