ETA: It depends on what scale and tone you want for the villainy, really: is it something that's going to threaten the nation, or harm a single person? Comic-book style melodrama or subtle creepiness?
(For the latter, how about having your shapeshifter murder someone and then replace them: shift to look exactly like them, move into their life, take over the murdered person's job, their partner, their house, their family ... and no-one will ever know.)
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ETA: It depends on what scale and tone you want for the villainy, really: is it something that's going to threaten the nation, or harm a single person? Comic-book style melodrama or subtle creepiness?
(For the latter, how about having your shapeshifter murder someone and then replace them: shift to look exactly like them, move into their life, take over the murdered person's job, their partner, their house, their family ... and no-one will ever know.)