I don't think Alexander saw conquest as a necessary evil. While I disagree with Paralus that his express goal was hunting around for free people to enslave -- all it meant to most average people was that the surplus value of their labour would be creamed off by a different elite -- I don't think it's plausible to doubt that the man loved to fight and loved to conquer. It was his life, and, in his mind, not only a worthy calling, but a divine one. And his people, in general, agreed; when they argued with him they never said anything like, "We should just live happily and peacefully within our borders because conquest is bad." The Persians didn't have anything against conquest as a concept either, else they wouldn't have had an empire; nor the various Greek city-states, as they'd worn each other out in wars as each tried to dominate the others. War as a noble calling, albeit sometimes tragic, is the key theme in the whole Homeric ethos, to which Makedonians unapologetically aspired.Obviously being able to do something doesn't necessarily make it right, but I would argue that much of Alexander's spending was centered on making what he saw as a necessary evil possible. That is, the continuation and fruition of the military campaigns that made the conquest of his empire possible.
With regard to the question of producing heirs, I was thinking about this further yesterday, and it really was a tricky thing for a Makedonian king, with several sets of Skylla and Charibdis to walk through. Obviously he should marry fairly young, so as not to risk getting killed in battle before siring heirs at all -- but who should he marry? There was always the risk, as Amyntoros pointed out, of offending one noble house by marrying into another. He could avoid this by marrying only foreign princesses to cement his victories and alliances -- something that was probably sometimes required. But then he risked offending all the noble houses by choosing none of them... and having people question the legitimacy of his heirs because they aren't full-blood Makedonian, as Attalos did with Alexander.
Then there was the question of, how many heirs to produce? Too few and you might end up with none, setting up for a succession war among other noble houses. Too many, especially from different mothers, and they might have a succession war among themselves. It wasn't as if the king could precisely control how many heirs he had, either; he might be infertile, one or more wives might be infertile, the babies might turn out to be stillborn, girls or defective boys, wives might die in childbirth, kids might die of diseases, etc. etc. etc. He had to make calculated gambles.
I think what a king would have wanted the most would be to produce several competent sons who were willing to work as a team for the good of the kingdom, rather than off each other, so that if the eldest were killed in battle, the second could take over, and so on. Alexander had, as an example, an ancestor who had done just that: his grandfather, Amyntas III. When his oldest son, Alexander II, died, his second, Perdikkas III, took over, and on his death the third son, Philip II, became regent for Perdikkas's infant son Amyntas IV, and was acclaimed king later. I think the fact that Philip didn't have Amyntas killed, when he might prove a threat on reaching manhood, bespeaks not only the practical need for male heirs, but affection for his late brother... which Alexander III, of course, didn't share, having never known him. Proving, of course, that even this relatively stable model had its dangers... it might even boomerang on the king himself if his tight team of sons decided to team up against him.
Still, it argued for choosing one main wife and begetting heirs from her; without being raised by competing mothers, they'd more likely get along. But then what if that one wife proved barren, or produced only girls, etc. ...? And so the thorny problem goes. History proves that sustaining dynasties in general, in cultures where kings risk their necks on the battlefield, is a very difficult thing, and depended very much on sheer luck. I'm sure it's one reason they eventually quit risking their necks.
The odds of ensuring a stable succession were tough enough for a king who survives long enough for his heirs to grow up; note that two out of three of Amyntas's sons died, so that one more death would have wiped the line out. For one who dies young, before his sons are old enough to build their own power-bases of followers... I wonder how many examples history can furnish us, of a fighting king of a warlike people who died 32 or younger and was succeeded by his progeny, whether they were infants or 12-year-olds on his death. I'm going to guess very, very few.
Karen