In this background, the reference really is a request to determine whether or not the national assembly resolution had any merit or whether it was a contempt of court. The reference also establishes the principle that the judgment of the Supreme Court has greater meaning and weight than a resolution of the national assembly. In order to understand the case it is important to know the background.

Here are some clippings from Nation and Express newspapers and Wikipedia:
Nawab Muhammad Ahmed Khan, Kasuri’s father, was killed in 1974. Kasuri himself was the complainant for the murder case registered against Bhutto, who was eventually hanged in 1979. In April this year, 32 years after Bhutto’s death, President Asif Zardari filed a reference under Article 186 of the Constitution to the Supreme Court to reopen the murder trial.
Bhutto was convicted in a murder case and sentenced to death by the Lahore High Court (LHC) in 1979 during the dictatorship of the then army chief General Ziaul Haq. He was executed on April 4, 1979 by then military dictatorship.

A five-member bench of the LHC, headed by Maulvi Mushtaq Ahmad, had held the Bhutto trial for five months and awarded death sentence to him on March 18, 1978. The Bhutto family had filed an appeal in the Supreme Court. A seven-member bench upheld his death sentence in its February 6, 1979, verdict with a bare 4-to-3 majority. His review petition was also dismissed on March 24, 1979. Bhutto was hanged at the Central Jail, Rawalpindi, on April 4, 1979.

Sheikh Anwarul Haq is a former Chief Justice of the Supreme Court of Pakistan [September 23, 1977 - March 25, 1981]. He is often considered ‘ill-famed’ for giving legitimacy to General Muhammad Zia ul-Haq’s martial law and for upholding the decision of the Lahore High Court which sentenced Zulfikar Ali Bhutto to death for the authorization of the murder of a political opponent. Four Supreme Court judges headed by Chief Justice Anwarul Haq upheld the murder conviction of Zulfikar Ali Bhutto. On 25 March 1981, S. Anwarul Haq became the first Justice and only Chief Justice to refuse taking the oath under the military imposed PCO and resigned on conscientious grounds.

Prime Minister Bhutto began facing considerable criticism and increasing unpopularity as his term progressed, the democratic socialists alliance who had previously allied with Bhutto began to diminish as time progresses. Initially targeting leader of the opposition Vali Khan and his opposition National Awami Party (NAP), also a socialist party. Despite the ideological similarity of the two parties, the clash of egos both inside and outside the National Assembly became increasingly fierce, starting with the Federal government’s decision to oust the NAP provincial government in Balochistan Province for alleged secessionist activities and culminating in the banning of the party and arrest of much of its leadership after the death of a close lieutenant of Bhutto’s, Hayat Sherpao, in a bomb blast in the frontier town of Peshawar.

Dissidence also increased within the Pakistan Peoples Party (PPP), and the murder of a leading dissident Ahmed Raza Kasuri‘s father led to public outrage and intra-party hostility as Bhutto was accused of masterminding the crime. Powerful PPP leaders such as Ghulam Mustafa Khar openly condemned Bhutto and called for protests against his regime. The political crisis in the NWFP (now Khyber Pakhtunkhwa) and Balochistan intensified as civil liberties remained suspended, and an estimated 100,000 troops deployed there were accused of abusing human rights and killing large numbers of civilians.

On January 8, 1977 a large number of opposition political parties grouped to form the Pakistan National Alliance (PNA). Bhutto called fresh elections, and PNA participated fully in those elections. They managed to contest the elections jointly even though there were grave splits on opinions and views within the party. The PNA faced defeat but did not accept the results, alleging that the election was rigged. They proceeded to boycott the provincial elections. Despite this, there was a high voter turnout in the national elections; however, as provincial elections were held amidst low voter turnout and an opposition boycott, the PNA declared the newly-elected Bhutto government as illegitimate.

All the opposition leaders called for the overthrow of Bhutto’s regime. Political and civil disorder intensified, which led to more unrest. Bhutto imposed martial law in major cities including Karachi, Lahore and Hyderabad. However, Mr. Bhutto accepted that there were major irregularities in the election in a number of constituencies and a compromise agreement between Bhutto and opposition to hold fresh election in some constituencies was ultimately reported. This compromise theory was however probably a later day addition as a major PPP armed rally was in the offing.

Zia planned a the Coup d’état carefully as he knew Bhutto had integral intelligence in the Pakistan Armed Forces, and many officers, including Chief of Air Staff General Zulfiqar Ali Khan and Major-General Tajammul Hussain Malik, GOC of 23rd Mountain Division, Major-General Naseerullah Babar, DG of Directorate-General for the Military Intelligence (DGMO) and Vice-Admiral Syed Mohammad Ahsan, were loyal to Bhutto.

To remove this intelligence, Zia secretly contracted with the active duty British SAS army officers to maintain a staff course for the Army personnel while Chief of Naval Staff Admiral Mohammad Shariff quietly removed naval personnel loyal to Bhutto and his government from the Navy’s active duty. Zia ordered Bhutto’s loyal officers to attend a staff and command course and none of the officers were allowed to leave the course until the midnight. Meanwhile, Zia with his close officers, including Admiral Mohammad Shariff, then-Chaiman of the Joint Chiefs of Staff Committee, arranged the coup in the evening. On July 5, 1977, before the announcement of any agreement, Bhutto and members of his cabinet were arrested by troops of Military Police under the order of Zia by the evening.

Bhutto’s last personal appearance and utterances in the supreme court were not merely a long defence of his conduct he also made some matters clear. He mentioned the words of “heir” for his son “Mir Murtaza Bhutto”. He made some remark which indicated that he has views similar to a Sunni, though he was Shia albeit a non-practicing one. He also effectively cast doubt on the reliability of star witnesses against him i.e. Masood Mahmood who was a UK-trained lawyer and not merely a police officer and FSF chief. He mentioned repeatedly Lahori Ahmedi connection of Masood Mahmood in his testimony. He repeatedly brought the subject of his maltreatment in the death cell. Bhutto made it abundantly clear, even though indirectly that he wanted either freedom or death, not something in between, and appreciated Khar and his lawyer Yahya Bakhtiar.

During Bhutto’s five years in Pakistan’s helm, Bhutto had retained an emotional hold on the poor masses who had voted him overwhelmingly in 1970s general elections. At the same time, however, Bhutto had many enemies. The [socialist economics] and nationalization of major private industries during his first two years on office had badly upsets the Business circles… An ill-considered decision to take over the wheat-milling, rice-husking, sugar mills, and cotton-ginning, industries in July of 1976 had angered the small business owners and traders. Both leftists— socialists and communists, intellectuals, students, and trade unionists— felt betrayed by Bhutto’s shift to centre-right wing conservative economics policies and by his growing collaboration with powerful feudal lords, Pakistan’s traditional power brokers. After 1976, Bhutto’s aggressive authoritarian personal style and often high-handed way of dealing with political rivals, dissidents, and opponents had also alienated many….