Richard should know better. Theoretically at least. But between everything happening and the pain he doesn't think, just reacts. He counters Damian's bite by wrapping his hand in the vampires hair and pulling him off. He uses a a good amount of his strength to do it too, leaving a how in his upper chest near his shoulder, not just in the shirt but in his flesh and muscle too.

The most imediate show that it was a stupid thing to do is that the arm pinning Damian's shoulder goes limp and he's now struggling to hold the vampire away from him just by that red hair.