I know this is a joke but it would be Crota because he killed thousands of Guardians. He also probably killed a ton of Vex, and other species.
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Bro, have you seen the -blam!- papa oryx did?
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As someone else mentioned, oryx conquered entire solar systems
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Oryx has decimated hundreds of planets over the thousands of years he has been around. Crota killed probably a few hundred or maybe thousand vex when he let them into the dreadnaught, and then a couple thousand guardians on the moon. Oryx's body count is in the trillions or higher