By the end of Darren Wilson's testimony, he volunteered an explanation for why he chased unarmed teen Michael Brown, who he describes as "like a demon." Earlier in his testimony, he tells in detail why he was terrified of the youth, who he claims beat him nearly to death in the police car.
In the screen shot below, you'll read that Wilson made sure the jury heard that he chased Mike Brown in order to protect his fellow officers who were on their way to back him up. So, he was terrified but not too terrified to chase the teen. He knew he couldn't physically subdue Brown, and he chased him in order to protect his back-up, which had not yet arrived, officers who had guns of their own and would have arrived as a pack.
I remain convinced that Wilson wanted to shoot Mike Brown. I won't explain that again in this post, my reasons are posted here.