It all happened so fast; he could process it, but he couldn't stop it. As Rossi grappled with the man--Rossi's leg something was wrong with his leg he needed help--Reid grabbed the gun that had fallen on the ground, bringing it up and pointing it at the UnSub, but then he saw Rossi stabbed.

It felt like the world slowed down. He shut off his feelings for the moment--if he panicked or screamed or lost it in any way, Rossi would die--and he struggled to get a clear shot on the UnSub. Aim, fire, and follow through.

It took him a moment to find the shot he wanted (though it felt like forever) and he pulled the trigger, praying that his aim was true and that Rossi would be okay.