This is a great clip from the episode "Masquerade." In it, Stefan makes it clear to Bonnie: they plan on killing Katherine. Check out the weapons they'll use.
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A special Halloween episode of The Vampire Diaries centers around a masquerade ball and a plan to kill Katherine. Check out the official preview now.
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The Vampire Diaries Season 2 Episode 7 Quotes

You aren't gonna kill Katherine... I am.

Stefan

Tell Damon and Stefan to give me the moonstone or I will rip this town apart until it rains blood.

Katherine