Forbes Magazine has a column by, well, Steve Forbes who reviews inter alia Paul Doherty's The Death of Alexander the Great: What or Who Really Killed the Young Conqueror of the Known World? Ecce:

Doherty makes an even more persuasive case for Alexander the Great's not dying a natural death. By the time Alexander reentered Babylonia after his withdrawal from India (a result of a mutiny by his army), his generals and courtiers realized that they were all increasingly vulnerable to the master's purges and that past service counted for nothing if Alexander turned against you.

Who killed him? Most likely Ptolemy, a noted warrior and Alexander's chief bodyguard. Ptolemy realized he'd climbed as high as he could in Alexander's service, and he'd seen firsthand how precarious his position--and that of everyone else--truly was. He probably conspired with Antipater, the general Alexander had appointed to run his affairs in Macedonia while he was out conquering the world. (At the time of Alexander's death, one of his armies was on its way to Macedonia to remove Antipater.) Ptolemy's position gave him access that no one else had to put poison--arsenic--in Alexander's wine. Arsenic would account for the fact that Alexander's body did not decompose in the stifling Babylonian heat. (Ptolemy had probably also poisoned Alexander's second-in-command, and rumored lover, Hephaestion, the year before.)

After Alexander's death Ptolemy went on to Egypt, where he put himself on the throne. Antipater was given a second lease on political life when the army sent to remove him was then put at his disposal.