My impression is that he was surprised by the quick police response. Some of the people running out the theater ran into the police coming in. He apparently kicked in a back emergency door and his car was parked in back. So he was probably going to drive away and escape, but the police were just too fast and caught him in his car. Apparently he told the police he may have had explosives in his apartment. If he is connected to the before mentioned Pakistani terrorist group, they are experts in explosives.
Those are steel doors in a steel frame. Tough to kick in. I doubt he planned this all carefully and just assumed he would be able to kick the door in.