The tension was so real for Will when he clasped the necklace around Hanna's neck. He didn't know whether it was enough to show his feelings for her, or if the message was delivered well. He mulled over a lot of possibilities and what he had to do to make it work.
However, that was also when he saw her coming closer, his mind froze and his first thought was; he was being attacked, thus his instinct told him to defend himself. Everything happened so fast. Even Will couldn't process the fact that Hanna was harmless. She was not an enemy and it was absolutely ridiculous and impossible for her to harm him.
Yet, his twisted mind read the whole thing differently because Hanna moved closer to him so abruptly.
The sight of her crowding him overlapped with another unpleasant memory and the next thing he knew; he had pushed her so hard, as if he was fighting for his life.