Embrace, extend, extinguish.
Of course no-one would complain if it wasn't for the fact that Microsoft has probably caused each individual slashdotter many many man-hours (months? years?) of pain and anguish trying to make a stupid DOS|Windows|Win NT|Win95|Win98|Win2000|WinXP|etc... machine work as it should do. Hell I remember how difficult it was to get the Network edition of Windows 3.11 to do TCP/IP!!! Even if we abandoned Microsoft for more reliable alternatives, it continued to cause us pain because of friends and family who needed our help and we couldn't refuse.
I repeat, if Microsoft had made a reliable fast product (I still feel sad that OS/2 died), no-one would be complaining. So, yes, given that Gates was personally responsible for running the company that made many many hours of our lives a misery with a thoroughly mediocre OS, I would say quite a few of us have a problem with him for good reasons.
And no, having made your money as a convicted monopolist, spending it in a philanthropic way does not remove the reasons to dislike him. I am glad that, having made billions of dollars, he is spending it in this way, but I would much rather he hadn't made it in the first place, and just maybe, we would have had a very different software industry based on sharing of code and tools that people in the third world could have afforded to join.