Well look at it this way. That Tsarnaev guy planned for some time to blow up as many people away as possible. He got up in the morning with the plan in mind, to execute the plan and kill as many people as possible. That is premeditated murder, Murder One.
This Cop likely had no thoughts of killing anyone when he started his day. He was just a doing his cop job when he stopped this guy for a bum taillight. Then the guy ran, the cop pursued
(good policy would have been for him to call backup), he caught up with the guy and a tussle ensued. Then the guy ran again, the cop shot him. Not with premeditation but in the heat of the moment. Yes he killed the guy, wrong as hell but not murder one, that's manslaughter. Put him on trial for Murder One and he get off. Better go Manslaughter.