Apart from the alleged ICC's "politicization" of Duterte's drug war, Roque said UN High Commissioner for Human Rights Zeid Ra'ad al-Hussein's recent remark that Duterte should take a psychiatric test also contributed to the pulling out of the Philippines' membership from the Hague-based court.
"The Government remains resolute in effecting its principal responsibility to ensure the long-term safety of the nation in order to promote inclusive national development and secure a decent and dignified life for all", the letter added.
Second, the country's withdrawal from the ICC does not absolve it from its unfulfilled obligations and from crimes committed while it was still a signatory.
Police have said they have killed about 4,100 drug dealers who violently resisted arrest.
The ICC can only intervene when a member state is unable or unwilling to carry out investigations and prosecute suspected perpetrators.
The president of the court's governing body, the Assembly of State Parties (ASP), expressed concern at the move by Manila.
It said the withdrawal from the Rome Statue was due to the "baseless, unprecedented and outrageous attacks on my person as well as against my administration" by United Nations officials, and what he said was an attempt by the ICC prosecutor to seek jurisdiction over him "in violation of due process and presumption of innocence".
The progressive lawmaker said the Statute also states that the withdrawal would not affect any cooperation with the Court in connection with criminal investigations and proceedings, and the withdrawing State's duty to cooperate in the said investigations which were commenced prior to the date on which the withdrawal became effective. Human rights lawyer Jude Sabio and opposition leaders Antonio Trillanes and Gary Alejano pressed separate and independeent charges of crimes against humanity against the Duterte administration.
Presidential Spokesperson Harry Roque Jr. defended move to repeal the Philippines' ratification of Rome Statute, a treaty that established the ICC, saying that Duterte could no longer take the alleged "politicization" of his anti-drug war, and the "conspiracy" between the United Nations (UN) and "pressured groups" to humiliate him.
As of October 2017, 123 countries, including the Philippines, were party to the treaty, which established four core global crimes: genocide, crimes against humanity, war crimes, and the crime of aggression.
"I would like to address myself to the International Court of Justice and to the prosecutors coming here to investigate: the war or the drive against drugs will not stop and it will last until the day I step out".
They said Mr Duterte's decision was an admission of guilt and a sign that he was panicking.
Last week, he said the ICC would "not in a million years" have jurisdiction to indict him.
The ICC tries people for "genocide, war crimes, crimes against humanity and aggression", according to its website.
The attempt to withdraw from the ICC, therefore, will not save anyone responsible for crimes against humanity from the ICC's jurisdiction, ICC argued. China has called on the global community to respect the Philippines' sovereignty on the issue.
Panelo insisted Duterte was not afraid, but objected to the ICC becoming "a political tool to harass a particular country, like ours".