According to his uncle Eddard Stark, the Wolfsblood was "strong" in Arya and Rickon, making them wild and nearly unrestrained. And the Targaryen side of his family was also known for their hot tempers. Both of those bloodlines had been strengthened in one way or another during his rebirth ritual. With an iron will, he slowly managed to take control of the pure unadulterated rage he was feeling.