You can't. KDR is kills divided by deaths. It's a ratio. 1/100 (assuming one person was shot to death 100 times) would produce a KDR of .01. Don't believe me? Jump into a random game and ask if people can organize it for you to die 100 times.
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"Negative" in FPSs means you got less kills then deaths. Not that your k/d is literally a negative interval.
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You can go negative genius. Meaning that you have more deaths than kills. KDR of .5 still technically positive is still negative.