The reasons that I hate Microsoft (and probably everyone else who does) have little to nothing to do with the video game market, although some of their behavior in the gaming market is typical of what people hate about them.
Competition is a great thing if the competition is based on producing the better product, which Microsoft rarely does. They're pretty much unable to enter any market without product dumping, and they've accumulated so much money from their Windows/Office monopolies that they can afford to continue dumping until they're the only ones left.
Of course, you can perhaps grant them some leeway now, since they've become such a huge company that one hand doesn't know what the other is doing. The huge amount of money they have on hand now allows them to be more pro-consumer than they would have been in the past (case in point RROD - TMB "typical Microsoft behavior" would have been to tell everyone with a broken console to go pound sand. If it were not a hardware issue, where certain laws about merchantability apply, they probably would have)
Of course, if it weren't Microsoft it probably would have been someone else (maybe IBM - if IBM had never contracted out the OS for their PC to Microsoft, they'd be nowhere today)
In the most personal sense, the reason I hate Microsoft is because they ruined computing. I used to enjoy it, and I was planning to be a CS major in college. I graduated from high school right at the time Windows 95 came out, and it was becoming more and more apparent that I would have to deal with Windows - and I hated it. Perhaps it was my own mistake (if I had stuck with it until now, I might have been able to work as a Linux/OSX programmer today), but I absolutely despised what a steaming pile of shit Windows was, and at the time it seemed like it was the only option. I dropped out of CS after my first year because I just didn't like computing anymore. I went to being a science major, and now I'd perhaps like to get back into it but now I don't have the time considering I've already chosen a different career path.