"The Israelis killed my niece last summer. Now I want to kill them," he told AFP after completing a week-long youth training camp with militants from the Ezzedine al-Qassam Brigades, the armed wing of the Islamist Hamas movement.
"I will become a resistance fighter," the boy said proudly during a graduation ceremony in Gaza.
Hatem is one of 17,000 youngsters who graduated late last month from two military training camps where Hamas – the de facto power in Gaza – said it was preparing the next generation to fight against Israel.
Last summer, Israel and Hamas militants fought a 50-day war that killed nearly 2,200 Palestinians and 73 on the Israeli side, and left swathes of the impoverished enclave in ruins.
It was their third conflict in less than five years, following an eight-day bombardment in 2012 and a 22-day war in late 2008 and early 2009.
Children were on the frontline of the latest conflict, with UN figures showing about 500 were killed.
And just five months after the war ended, thousands of those who survived signed up to join the Hamas training camps.
"I want to join the Qassam Brigades because they are the strongest in Gaza," said 15-year-old Mohammed Abu Harbid, who also took part in the training.
With the humanitarian situation in post-war Gaza growing steadily worse, a fresh flare-up with Israel seems likely.
And with many schools being used to shelter the displaced and unemployment standing at 41 percent, it was not difficult to convince youngsters to join the camps.
Hamas insists that teaching children how to fight is a legitimate part of "resistance" activities against Israel.
But the camp, which accepts boys and men aged 14 to 21, has incensed human rights activists who accuse Hamas of exploiting children traumatised by war.
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Hamas has been running summer camps for youngsters for years, but this week-long session was a much more serious affair.
Run for the first time by militants from the Qassam Brigades, there were no "fun" sessions –- and no mid-week visits to Gaza's zoo.
"They were trained intensively in using light and heavy weaponry and were taught how to ambush, so they can lead the next battle for liberation," the Brigades said on its website.
Hamas has rushed to defend the military training.
"The Western media accuse Hamas of militarising society with their training camps, but what has the West done to stop the enemy from carrying out its crimes?" senior Hamas official Bassem Naim said.
"What have we gained from 20 years of futile negotiations?"
The latest round of US-led peace talks between Israel and the Palestinians broke down in April 2014 and the war in Gaza has further distanced any hope of a return to negotiations to end the decades-long conflict.
"We are an occupied people and international law guarantees us the right to resist," Naim wrote on Facebook.
But local human rights groups are accusing Hamas of exploiting children for political purposes.
"We are not disputing the right of an occupied people to resist, but it must be done by adults, not children," one human rights activist told AFP, speaking on condition of anonymity.
"The camps are making young people aggressive instead of educating them and teaching them to abide by the law," the activist said.
Issam Yunis, head of Gaza-based human rights group Al-Mezan, said the camps were a dangerous development in a territory where more than half of the population is under 15.
"Gazan children are traumatised by the violence, so some are attracted by the military training," he said.
"But the priority today should be to take care of their social and physical well-being."