Two takes on why Morsi did what he did
Tarek Masoud, writing for CNN (not his headline, btw), argues an Egyptian tradition of a strong executive is what counts:
But, as shocking as Morsy's actions are, they do not prove that Islamists cannot be democrats. Morsy's decision to grant himself unquestioned authority was not the final, spectacularly public phase in some hitherto clandestine Muslim Brotherhood plan to erect a holy autocracy. Instead, the Egyptian president simply did what Egyptian presidents have been doing for more than 60 years — that is, loosening institutional restraints on their authority in order to more easily fulfill their agendas.
That Morsy is an Islamist is largely irrelevant. It's likely that the autocratic temptation would have seized Egypt's president regardless of his party or ideological orientation. This is not only because Egypt has had a distressingly long history of powerful executives, it's also because, at this moment in Egypt's political history, there is no actor, institution or organization able to keep the presidency in check.
Steve Cook, writing in Foreign Affairs, says it's an attitude to governance shared by the MB and the Free Officers:
The Brothers, like the Free Officers who came to power in 1952 and produced Gamal Abdel Nasser, Anwar Sadat, and Mubarak, are what the Yale anthropologist James Scott calls "high modernists." High modernism, which places a premium on scientific knowledge and elites with special skills, is inherently authoritarian. It might seem a strange designation for the Brotherhood, since most observers think of it as a religious movement. But in reality, the group has used religion to advance a political agenda. To suggest that the organization's leaders are dilettantes when it comes to Islam would be an overstatement, but the majority of them are first and foremost doctors, lawyers, pharmacists, and engineers. They think of themselves as a vanguard that is uniquely qualified to rebuild Egypt and realize its seemingly endless quest for modernization. Moreover, they believe that the people entrusted them with the responsibility to do so as a result of free and fair elections in late 2011 and 2012.
With the Brotherhood in control of the now-dissolved People's Assembly, Shura Council, Constituent Assembly, and the presidency, this vanguard thought it could choose a path for Egypt within the councils of its own organization. There was no need for consensus or negotiation, hence Morsi's August 12 decision to decapitate the national security establishment and his subsequent efforts to place sympathizers in influential positions within the state-controlled media. In a television interview broadcast on November 29, he even called his recent decree an effort to "fulfill the demands of the public and the revolution." There is, he implied, no reason to question his decisions, which were in the best interest of Egypt.
I must say I lean towards Cook's "high modernist" interpretation because this is how the MB has behaved since the revolution: it's excited to be in a position to implement its project, makes a big fuss about its Renaissance Project, and sees others as saboteurs of that perfect plan. But where I disagree is with the decapitation of the SCAF: the SCAF decapitated itself and enabled Morsi — who had no power to do any of this without the consent, tacit or explicit, of the military.