Set in Stone

Merlin has no reason whatsoever to trust Mordred, and more than a few reasons not to. He's seen the signs, been warned more than enough times, and all the energy he's wasted in the past trying to thwart the Dragon's prophecies pretty much just proves that the future is already decided. Destiny is painting Mordred as the villain, and so far, it's not been wrong.
Distrusting him is easier said than done, though, particularly when half of Camelot seems to think Merlin's hopelessly infatuated with him.