Over the course of the a year ago, police have insisted, often contrary to evidence, that those killed in police operations "fought back", usually by firing at - but invariably missing - police.
He added that the Philippine government must remember that it has obligations under global treaties on the respect for human rights.
The boy's death has fueled public outrage, denouncing the president's bloody war on drugs which has already killed thousands of drug suspects since June 30, 2016.
Duterte, who promised to protect policemen involved in his intensified war on illegal drugs, said the killing of Delos Santos should be investigated to determine if law enforcers have criminal liability.
Delos Santos was found dead, face down on a garbage heap.
A number of Duterte's powerful allies in the senate have broken ranks and signed a resolution condemning "the recent spate of abuses by police resulting in excessive and unnecessary deaths in the conduct of the campaign against drugs". In the election his campaign focused on a promise to use deadly force to wipe out crime and drugs.
Protesters hold placards and lit candles at the wake of delos Santos.
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Allegedly, it was Oares who shot and killed the victim. He said he does not mind being ousted over his crackdown on illegal drugs.
The Karapatan officer batted for "an entire overhaul" of the security apparatus instead of "human rights trainings which goes out the right ear after they hear it in the left", and "an end to the drug" to give way to a "comprehensive solution to the illicit drug trade that treats the problem in its economic and social roots".
The Catholic Church spoke out against the killings in a Sunday mass, calling on police and vigilante gangs to "stop wasting human lives". "It is a humanitarian concern that affects all of us".
Duterte's initial response to the spike in killings that saw 91 people gunned down in seven days was to encourage more bloodshed and offer to promote those police officers responsible.
In one province, Bulacan in the central Philippines, police killed at least 32 people in the span of 24 hours last week.
Surveillance footage showed 2 officers accompanying Delos Santos to the spot where the boy was killed.
"Kayong mga pulis kayo, you will find that you are far much worse than drugs, because you are destroying the credibility of government", Duterte said.