According to Michael Hudson, Soleimani was murdered because he would have been instrumental in Iraq’s quest to control its own oil. From Michael Hudson at counterpunch.org:
The mainstream media are carefully sidestepping the method behind America’s seeming madness in assassinating Islamic Revolutionary Guard general Qassim Suleimani to start the New Year. The logic behind the assassination was a long-standing application of U.S. global policy, not just a personality quirk of Donald Trump’s impulsive action. His assassination of Iranian military leader Suleimani was indeed a unilateral act of war in violation of international law, but it was a logical step in a long-standing U.S. strategy. It was explicitly authorized by the Senate in the funding bill for the Pentagon that it passed last year.
The assassination was intended to escalate America’s presence in Iraq to keep control of the region’s oil reserves, and to back Saudi Arabia’s Wahabi troops (Isis, Al Quaeda in Iraq, Al Nusra and other divisions of what are actually America’s foreign legion) to support U.S. control of Near Eastern oil as a buttress of the U.S. dollar. That remains the key to understanding this policy, and why it is in the process of escalating, not dying down.