Sometimes, it's best to listen to your father. Dexter should have killed Arthur Mitchell soon after he met him.
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Trinity comforts his daughter in this scene from Dexter, soon after he learns that she knows exactly who he is.
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Dexter might be a serial killer, but he's also now a single father. (RIP, Rita.) How will that affect his murderous habits going forward?
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Turns out, Christine is more than a nosy reporter on Dexter. She's the daughter of the Trinity Killer.
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Dexter Season 4 Episode 10 Quotes

I know that look. I'm not the only one on a stalk.

Dexter

Dexter: He's never taken a child before.
Harry: That you know of. If this is new, it's on you.