Interestingly, the most common contexts which I often hear this word in are just about all positive. This short story was written some half a century ago, but I think the usage stays the same. It tells us the girl is playful and possibly teasing.
狡そうな眼をしてみせて
And she is doing such a face and showing her cunning eyes on purpose!
ずるい is very commonly used between people romantically involved, as in . 川端's text. It is just something that could sound very flirtatious in the right context. Read this:
「彼女と付き合う前のデート。日も暮れていい雰囲気になってきたので、隙を見て抱き寄せ、キスをしました。すると彼女は一度離れ、照れたような顔をして、“ズルい”と言ったんです。一瞬ダメだったかな……と思ったんですが、彼女はまんざらでもない様子だったので、思いっきり抱き付きました」アツシ(仮名)/30歳
この場合、「ズルい」という言葉にネガティブな意味はありません。むしろ、キスに対する感想としては最高の言葉かもしれません。 照れたような表情も加えることで、男性をさらにドキッとさせる効果があるようです。
This is probably a little personal but there was a period of time when I was told that I was 「ずるい」 almost on a daily basis, and it made my heart sing every time I heard it. Well, happy memories of days gone by.
Well, just for context and comparison, "sly" isn't necessarily a bad thing to say about someone; it is often employed to describe someone as showing signs of being adorably clever or withholding secrets. This is David Thomson speaking of Nicole Kidman
and Batman Forever, where she played a shrink, Dr. Chase Meridian, and did pretty well being cute and sly in a franchise film where the heavy lifting and the big money were kept for Val Kilmer, Tommy Lee Jones, and Jim Carrey.