Gerard was far too close to Nate, and while William didn't have a death wish, the odds of Nate taking Gerard down - given a little time and breathing room - were much better than William attempting to do the same. Nate knew Gerard better, and he was a war veteran besides. He was the one with the most potential experience fighting dark creatures. He'd already done more to distract Gerard than William had managed. He was also pinned down with no way out. Without a diversion, he would die at Gerard's hands (and teeth) and William would be a breath behind him.
William dropped out of the tree.
He hit the ground and was running a half-second later, as soon as his lungs remembered how to breathe and his body recovered from the shock of impact with the cold, hard ground. He just needed to give Nate more time, needed to distract Gerard, needed to get to the willow...
He slammed full-tilt into someone who appeared out of nowhere, out of the shadows, and gasped as he twisted away. "Jon?" He didn't have time for much more than that, though, and especially not time to look back. "Run!"