Hundreds of men attacked journalism student Mashal Khan last Thursday, stripping, beating and shooting him before throwing him from the second floor of his hostel at the Abdul Wali Khan university in the northwestern town of Mardan.
Khan had been known for his liberal views, especially on Facebook, sparking the blasphemy allegations against him.
Twenty-two people have been arrested so far over the killing, which came after the government intensified its rhetoric against blasphemy.
One university employee threatened to kill Khan and cut him into pieces, Hamdard said. Then the mob began kicking in the door to a washroom where Abdullah had taken refuge.
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"All this happened in seconds. They broke the door, some of them had batons, they were furious - suddenly they entered inside... They were not listening to anyone," Hamdard said.
Police arrived and managed to yank a wounded Abdullah to safety, he told Geo. At the sight of blood and the crowd, he added, "I lost my courage."
"They said, 'You are a non-believer, you have hidden a blasphemer'... They were crazy, they were not listening to me.
"Two of them kicked me and snatched my mobile and locked me in my room."
Hamdard was rescued by another teacher and spirited away by police. By then Khan, who had been hiding in his own room at a nearby student hostel, was dead.
"Mashal was a Diya (lamp). They have turned off a lamp," Hamdard told Geo.
He apologised to Khan's parents for failing to protect their son and said his guilt had driven him to resign.
Today an opposition leader Imran Khan, whose party controls Khyber Pakhtunkhwa province where Mardan is located, vowed the perpetrators would face justice.
The killing came in the wake of a government drive against blasphemy, a hugely sensitive charge in conservative Muslim Pakistan.
Unproven allegations have led to dozens of mob attacks or murders since 1990.
Last month Sharif swore that blasphemers on social media would be prosecuted. The interior ministry also threatened to block social media websites with blasphemous content.
Abdullah, who has been taken into protective custody, claimed the university had wanted him to brand Khan a blasphemer because he (Khan) had criticised the school, according to court documents released late yesterday.
A second student told a magistrate he had testified before a university "congress" on the day of the killing, accusing Khan of blasphemy, according to the documents seen by AFP.
Rights activists have long criticised the colonial-era blasphemy legislation, saying it can be abused for personal vendettas.