Since police won’t give us a motive, lets come up with our own.
What I think is most likely:
Guy up for days on meth who thought people were trying to kill him. Loaded up his car with guns and took off to get away from the people he thought were trying to kill him. Gets pulled over by police, thinks they are going to kill him, so shoots the cop and takes off, killing everyone he sees along the way in his meth induced psychosis.
Less probable but more interesting:
Guy was on his way to commit a mass shooting at the theater when he was pulled over. Guy decides his original plan is going to be stopped by the police so he shoots the cop and heads to theater killing as many people as he can along the way.
What do you think?