@talk72000,
Alexander certainly had a screwed up childhood, but i don't happen to have a very high opinion of him. Philip was pretty screwy, but not really that odd by the standards of his day. I ascribe most of Alexander's success to the military organization created by Philip, the officers trained by Philip, and the excellent logistical system which he had already put in place for the Persian expedition before he was assassinated.
By the way, although i should have done it before i posted, i did later check several of the near contemporary sources for the lives of Philip and Alexander (i know of no reliable contemporary sources), and it is entirely possible that Olympias murdered Cleopatra Eurydice, rather than that she committed suicide, and even that Alexander conspired in that. There is nothing like general agreement about these events among ancient sources, so i probably should not have spoken wish such an air of authority.
Given what we know of Epirus, and the likelihood that Philip relied upon Epirote officers and levies, the marriage to Olympias may well have been the most important marriage he contracted. In terms of their military culture, the Epirotes were every bit as formidable as the Macedonians. A couple of generations after the death of Alexander, Pyrrhus, the King of Epirus, was contracted by the Italic League--the Greek colonial city states of the south of what is now Italy--to deal with the Romans, who were then threatening the autonomy of those cities. He defeated the Romans several times, although at great cost to himself--the term "Pyrrhic victory" means a battle won at a cost to the victor greater than what has been accomplished. After the battle of Heraclea, in which the more extravagent ancient accounts have him destroying the equivalent of a full consular army (about 12,000 men), he was said to have looked on the Roman dead, all lying in windrows in their former lines, facing the army which had vanquished them, and wept, saying that with an army such as that, he could conquer the world. He went on to conquer and briefly hold Sicily (for about three years), but the Gauls overran Macedonia, which was to have protected Epirus in his absence, and he was an unsettled man, constantly eaten up by dreams of conquest, and he eventually died fighting the Spartans and the Argives in the Argolid.
Nevertheless, three generations before the birth of Pyrrhus, the marriage of Philip to Olympias would have secured a valuable ally for Macedonia, and at the least would have secured his only threatened flank on the eve of his invasion of Greece. It is not certain, but many modern historians believe that the Epriotes were an important part of Philip's military machine, both in terms of troops and of capable officers. Not every Epirote was a Pyrrhus--reckoned by many writers of antiquity to have been one of the greatest generals of the antique world--but they were good material.