I have to admit, Vahlen has never struck me as outright villainous, just a little too eager to see where her research takes her. The methods of interrogation are brutal and she takes too quickly to some of the research, but in game the justification is that humanity's back is against the wall and any advantage is worth the cost. But even she had limits, such that she was appalled by some of the gene mods that EXALT was using, refusing to allow XCOM operatives to be changed in the same fashion. The comparison to Halsey is apt, as she's not so much evil as she is convinced that what she's doing is right, the consequences be damned.

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CdrMike, Overwatch Reject