Going into the trunk would have taken him to crime scene #2, which many people here in FR don't comprehend. Most people do not survive crime scene #2.
He said in a news conference, that he had so much stuff in his trunk, that he couldn't have gotten in it, had he wanted to, which he didn't. He pretty much knew how that story would likely end, so he ran. Got pistol whipped for it, but 15 stitches is a small price to pay, given his immediate circumstances.