I don't think anyone is saying that the Light is evil, except someone who thinks in such absolute black and white that they believe the Light being able to be represented in evil form makes the whole thing evil. Which runs against the whole point of the story, doesn't it?
I'm not sure love is entirely alien to the Void as a concept, but they probably have a pretty shallow conception of it. I think Old Gods have shown they know that people fear having their loved ones taken away, but plenty of people in Warcraft have been willing to kill and betray their loved ones. Love is probably just another "invisible chain" to them, or something they'd interpret more as an attachment to one's belongings. An Old God would probably still be angry if you killed or stole its favorite pet.
And that's the thing. In a world where people have betrayed everything they stood for, killed almost everyone they ever loved, or worse, the Void simply can't understand why Alleria would adamantly reject so many of their paths for the sake of a child she doesn't even know. Some people would kill their fathers for less, so the Void thinks anyone would do the same if given sufficient motive.