A hero is defined as someone who is admired or idealized for courage, exceptional achievements or noble qualities. The question is, does Jack meet any of the requirements to make him a hero? Let's look at each aspect and see if he measures up.
Courage. Jack seems to possess some courage in the sense that he is willing to venture into the unknown to find prey, to hunt, albeit in the company of others. However, when he has the first opportunity to kill a living thing, he lacks the courage to strike, as illustrated in the follwing extract:
There came a pause, a hiatus, the pig continued to scream and the creepers to jerk, and the blade continued to flash at the end of a bony arm.
... Jack’s face was white under the freckles. He noticed that he still held the knife aloft and brought his arm down replacing the blade in the sheath. Then they all three laughed ashamedly and began to climb back to the track...
He makes the excuse that he was looking for a place to stab, but it was the thought of killing which overwhelmed him and he lacked the heart to carry it through. This embarrassment, however, does not deter him and he is later successful at achieving his goal. The question arises whether we should actually consider if Jack's inner savagery was what ovewhelmed him instead of sheer courage. His desire to hunt was not borne from a noble purpose to find food so that everyone could eat. There was more than enough edible vegetation on the island for the boys to survive. He wanted to have fun and, furthermore, exercise his bloodlust, dominance and savagery.
Furthermore, Jack's bravery is offset by the fact that he bullies Piggy, who is weak and vulnerable. He literally does not 'pick on one his own size' but targets someone he knows cannot retaliate. Although he is involved in confrontations with Ralph, these are emotional incidents, spurred by anger and frustration. In the final chapters, he employs the boys to hunt Ralph down since he needs them because he lacks the courage to do so alone. Therefore, all things considere, Jack falls well short in this regard.
Except for hunting and killing a pig, there is nothing exceptional about what Jack has done. His desire was selfish and self-serving. Although the boys at one point showed their admiration for him when he apologised to Ralph for allowing the signal fire to die when a ship passed in the distance, there was nothing significant and memorable about much else he did. His hunters followed his instructions because, firstly, he had been their leader from the beginning since he was head of the choir and secondly, they shared his sentiment. They wanted to enjoy their freedom and do much as they pleased, which was to hunt instead of being bogged down by rules and responsibilties as Ralph and Piggy demanded. In this regard then, Jack also failed.
Finally, Jack lacks noble qualities. In fact, his actions speak only of savagery and malice. He intentionally sets out to kill pigs to satisfy his lust for blood. He hides his identity underneath a painted face and assumes a new persona, he bullies and hurts Piggy, steals his spectacles, imprisons the twins against their will, hunts Ralph with the assistance of others, does not care much about rescue, just wants to have fun, refuses to take responsibility for tasks which are meaningful such as building shelters and taking care of the signal fire, encourages others to kill and hurt. These are definitely not noble attributes.
So, all in all, Jack Merridew is definitely not a hero.