Well, I like her in the sense that I enjoy that she's an evil unpleasant bastard. Just because a character isn't someone you would like in real life doesn't mean you can't appreciate them being in fiction, hence why people have stuff like "favorite villains" in the first place. I don't think just because a character in a fictious universe is cold, evil and unpleasant you automatically have to dislike her as an out-of-universe consumer of the media.
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As for why people rush to defend her, I'd say it's because her descent to the point beyond redemption was so gradual and subtle. Most "good gone evil" characters in Warcraft have a fairly quick heel face turn, where as she didn't.
All the way up until Wrath, you could honestly argue she was a well-intentioned extremist, because her (supposed) motives were sympathetic and she didn't commit so many more war crimes than anyone else in power. That's not to say she was unambigiously good or anything along those lines, but she was relatively easy to sympathize with. Come Cata, she started crossing a few more lines, but her motives were still ambiguous and you could still read into it as doing the best for her people. It wasn't until her faction leader short story that we as players were able to know that she had undeniably gone beyond the moral event horizon. Up until that point both sides of the debate had reasonable arguments, and this debate has been going on since Frozen Throne in 2001.
Basically what I'm trying to say is, Sylvanas has been one of the most debated Warcraft characters for so many years because she's always been morally ambiguous up until recently. People still defending her motives is just the aftermath of that.