MORE EXPLANATIONS: Here's an outstanding analysis of the reasons behind the Israeli attack on Shehade.
Also, I cannot believe that anyone is taking seriously the reports that had Israel not attacked, a cease-fire would have been imminent. As
Amos Harel writes in Ha-aretz:

Regarding the timing, defense officials indignantly rejected the suggestion that the assassination was deliberately timed to disrupt a possible Tanzim initiative for a unilateral cease-fire. They noted that Hamas was not a party to this idea in any case - and certainly Shehadeh, the organization's most extreme member, was not involved.
But even the Tanzim was far from deciding to adopt the idea, they added. Mohammed Dahlan, the former head of the PA's Preventive Security Service in Gaza, did propose it, the sources said, but his influence on the West Bank-centered Tanzim was small, and the defense establishment had received no information indicating that the Tanzim had decided to accede to this suggestion.
Palestinian sources also admitted that the initiative was not making much progress, "since it would be necessary to corral 30 local cell leaders into it." A senior defense official said Palestinian reports of the initiative were largely psychological warfare.