Davao City (CNN Philippines) — President Rodrigo Duterte said he was not surprised by the testimony of witness Edgar Matobato in the Senate probe on alleged drug killings.

However, Duterte said his son Paolo, who is Vice-Mayor of Davao City, was angered by Matobato's "lies".

Matobato, a self-confessed (former) hitman, testified during the Senate inquiry on extrajudicial killings last week that he killed people in Davao City upon orders of Duterte, who was city Mayor at the time.

Matobato also claimed there were instances when Paolo Duterte ordered the Davao Death Squad to kill his enemies. In response, the younger Duterte said Matobato's statements were the "accusations of a madman."

President Duterte said Matobato committed perjury by lying under oath. He added, the person who induced Matobato to testify may also be charged with violating the law.

"If you know it's a lie and you egg a person to testify, you are an active participant. 'Sige, sige mag-testify ka [Go ahead and testify]' that is subornation of perjury," Duterte said on Monday when he visited victims of the Davao City blast.

Watch: Alleged Duterte hitman faces Senate

Duterte: I do not intervene

Amid claims of Sen. Leila De Lima that the President was behind the plan of unseating her as justice committee chair, Duterte said he has nothing to do with it. He said he doesn't intervene with the affairs of the other branches of the government.

"I don't intervene. I do not want to dwell into matters not within my province,outside my territory. Basta ako sabi ko [I said], I don't care how many hearings there-- UN, EU. Basta ako nakatutok [I am merely focused] to my promise to the Filipino people that there will be no corruption," said Duterte.

De Lima, who chaired the hearings for extrajudicial killings and who presented Matobato as witness in last week's Senate probe, was ousted on Monday after 16 senators voted to remove her as the justice committee chair. De Lima believes Duterte was behind this.

"He was not concealing 'yung galit niya sa akin [his angst towards me] when I started this inquiry na talagang binastos ako ng husto [that was I was completely disrespected]," De Lima said.

The Senate probe on extrajudicial killings resumes on Thursday.

CNN Philippines Correspondent Triciah Terada contributed to this story.