The speaker only mentions his mistress' "cruel eye" once in Sonnet 133, but it's a pretty striking image. Plus, women's eyes are always a big, big deal in love sonnets like this. (If a woman is pretty or sweet, her eyes get compared to stars and beams of light. If she's in a bad mood or just can't stand the guy who's in love with her, her eyes will shoot daggers or lightning bolts. Sometimes, they'll even freeze a dude out with their icy stare. Impressive, no?) In Shakespeare's Sonnet 133, the mistress' "cruel eye" becomes a metaphor for how badly she treats the speaker and how awful it makes him feel.
Line 5: This is where the speaker says "Me from myself thy cruel eye hath taken." Since eyes can't technically be cruel and they can't do stuff like take people away from themselves, we know the speaker's using personification to make a point.
What point is that? The speaker is basically saying that his mistress is so cruel that she makes him feel like he's not even his own person any more. Or, maybe her cruelty makes him feel like he's going nuts. It could also be a reference to how he feels captivated by her physical beauty, despite her cruelty.
Okay, fine. Why does he say her cruel eye makes him feel this way? Probably because she's always giving him dirty looks that make him feel like garbage. Ever heard the phrase "if looks could kill"? When our speaker looks into his mistress' eyes, we're guessing he doesn't find any pity or love there.
If you want to see how another sixteenth-century poet writes about his mistress' eyes, check out Sonnet 48 from Philip Sidney's Astrophil and Stella.