You don’t know that. The police were on the way... He could have sat down and waited.
No, the whole point all along was to kill this guy. The victim may have known it when he saw them. Unfortunately, for the father, he wasn't smart enough to pull this off. But he didn't end up telling this many lies by accident.
He relied on the fact that his buddy was a D.A., and it made him stupid. Unfortunately, for him, the police get mad when a D.A. is resisting their case, and they know it's not personal considerations. Eventually, they can often go around the D.A.
How did he know the police were on the way and how did he know that if he just sat there they wouldn't shoot him?