He doesn’t need to be dropping bodies or shedding blood to cause a great deal of trouble.
From his position within the agents’ midst, Talbot — through no fault of his own — could ruin absolutely everything.
As far as they know, they have no reason to doubt or fear Talbot.
The brainwashed Talbot could do a lot of damage to the good guys at the Lighthouse before they even begin to suspect that anything could be off about him.