Why was Zulfikar Ali Bhutto a threat to General Zia?
Zulfiqar Ali Bhutto, after he was released after first detention enjoyed great public support, he traveled from one part of the country to another and addressed people; the same people that supported the grand PNA (National Alliance of 9 Opposition parties) were now rallying around him.
On the other hand was a military dictator with lust of power and new-found luxury that he wanted to enjoy enough before he could be removed, Bhutto was a big threat to his rule.
He had promised elections within 90 days, and he had to fulfill it sooner or later to maintain moral support and Zulfiqar Ali Bhutto was doing homework as a prospective successor. So, Zia had to get rid of him, to rip people off a popular option. He arrested him, released him, and then again arrested him with a stronger accusation to face, until at last he found a dummy in form of Ahmad Kasuri, who encashed the death of his own father.
Irony is, Z.A. Bhutto’s involvement in murder was testified by witnesses who were kept in detention until they confessed. “It was a mock trial in a Kangaroo court” said former US attorney General Ramsey Clark.