MANILA - Senator Leila de Lima should explain why she spent the whole of Thursday's hearing to hear the testimony of a self-confessed assassin who worked from 1988 to 2013, the Senate president said on Friday.

Senator Aquilino 'Koko' Pimentel said, however, that he will not call out the Senate Committee on Justice and Human Rights, which De Lima heads, for being out of bounds.

He would rather that De Lima "defend her decision why she spent one whole hearing hearing this witness."

"She should now probably justify not only to herself, but to her committee members and to the rest of the senators why this is relevant to her own proposed Senate Resolution no. 9," he told ANC's Dateline Philippines.

Edgar Matobato told a Senate hearing on Thursday that President Rodrigo Duterte, then mayor of Davao City, commissioned a hit squad called the "Lambada Boys" to kill among others, four bodyguards of a rival politician, a hard-hitting radio anchor, and the president's sister's lover.

Malacañang has denied Matobato's claims.

READ: What a self-confessed DDS hitman told the Senate

For Pimentel, Matobato's testimony is "totally irrelevant to proposed Senate Resolution no. 9." He has rejected the recommendation that Motabato be given security by the Senate because, he said, the upper house doesn’t have a witness protection program, and "if ever we should do so, it must be for a witness which has testimonies relevant to a pending matter in the Senate."

"Actually, kung nakikita natin, for those who followed yesterday’s hearing, it looked more like a trial—people were waiting for the cross-examination, etc. This is no longer in aid of legislation," he said.

Pimentel also noted that the Committee on Justice has the most number of resolutions or referrals and with one hearing for the proposed Senate Resolution no. 9 going wayward, its attention will be divided.

"Uubusin ng Committee on Justice and oras niya at atensyon niya dito sa proposed Senate Resolution no. 9 na lumiko na sa 1988 to 2013. Ano pa po ang maa-achieve ng Committee on Justice?" he said.

[The Committee on Justice will waste its time and attention on this hearing for the proposed Senate Resolution no. 9 which has has now gone back to the time of 1988 to 2013. What else will it achieve?]

Pimentel recalled, when the Senate investigated the Makati City Parking Building 2, the Blue Ribbon Committee was in charge. The Blue Ribbon committee, according to Pimentel, holds hearings "in aid of legislation and [for] accountability."

"Ito pong Committee on Justice, bills and [resolutions] about our justice system, about our human rights regime; bills and resolutions to improve the said systems—justice and human rights. Ang accountability po, wala na po yan sa jurisdiction ng Committee on Justice and Human Rights," he said.

[The Committee on Justice is for bills and resolutions on the improvement of justice and human rights. Accountability is not within its jurisdiction.]

Pimentel leaves it to the plenary if De Lima will be called out of order, but he said he will speak with her and the members of the committee to remind them of the Senate rules.

"We are all grown-ups, so we should read the rules and try to conform our conduct and our language pursuant to our rules, our customs, and our accepted practices," he said.

He added, it is good that there is difference of opinion among senators as displayed by Senators Alan Peter Cayetano and Antonio Trillanes IV, but reminded that there is a proper way to do it.

WATCH: Cayetano, Trillanes clash in Senate hearing

"We should be glad that there is a clash of ideas, a clash of positions, but let us avoid the intrusion into the physical spaces of the senator. Mayroon dapat, there should be a field of space where a fellow senator should not intrude anymore," he said.

"Kung mayroon kang reklamo that somebody is dominating the discussion, it’s correct to bring it now to the attention of the chair. You wait for the chair to rule and you let the chair implement her ruling. You do not implement your own ruling," he added.