I don't know why people have the impression that this guy actually had some genuine "Goal" here. He wanted to shoot people, found an excuse, and subsequently chose an easy target that was vaguely in-line with his alleged "motive".
He didn't choose something like a police station, political rally, or what-have-you because those are harder targets than a run-down supermarket. (Yes, I would consider an abortion rally as a comparatively "hard target". I'd expect they'd have some amount of police presence nearby to monitor the situation.)
Use some critical thinking. The implication is that if he is alive then he has the option to escape, because he is capable of acting due to not being dead. He isnt suggesting they'd leave the back door unlocked in the prison, nor is he suggesting there's a realistic chance of success.