When will Stefan ever not be useless?
Stefan knows Damon is still under the control of the Siren in some way and deduces that he needs to put Damon down for his own good and for the good of the gang. He achieves his mission, then goes of to play in bumper cars and leaves Damon unguarded, only for Sybil to come and rescue him 5 minutes later. Yes Stefan thinks Sybil is all locked up in the armory BUT he knows there is another Siren out there even he if he doesn't know who it is.
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Never argue with stupid people,they will drag you down to their level then beat you with experience.