I questioned this myself and realized very quickly how OP it would be in crucible.
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Here's the description for lifestealer: three rapid kills in succession will heal a small amount and start shield regeneration. That would hardly ever come up in Crucible, and even if you did mange to get three quick shotgun kills, the ability only gives you a small amount of health and starts recharging your shields. It wouldn't be overpowered at all.
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