The best replacement for backstab is mutilate.

The best alternatives to backstab are rated as following:
★★★★★  mutilate
★★★★★  hemo
★★★★★  ambush
★★★★  hemo 4.3
★★★★  hemorrhage
★★★  sinister strike
★  hemo spec
★  hemo pvp
★  shadow guardian
  ambush wow
  ambush rogue
  double sneak attack skyrim

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Speaking about backstab, one probably shouldn't forget about backstab.

Other search terms related to backstab and their ratings are:
★★★★★ backstab
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backstab
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