2nd Degree murder is a necessity. Every action has a set of motives. If someone kills someone by plotting for days how to do it and then executes that plan, that is 1st degree murder.
It carries more weight than someone who in the heat of passion, kills someone.
That is 2nd degree murder. You don't get a free pass. You can go to jail for life. You still are a murderer. You just did not premeditate and plan your action.
First-degree murder: any intentional murder that is willful and premeditated with malice aforethought. Felony murder is typically first-degree. Second-degree murder: any intentional murder with malice aforethought, but is not premeditated or planned.
The burden of proof is on the authorities to prove premeditation. That's a call for the DA to make. The charges can later be changed if he doesn't think it is likely that he can prove premeditation.
The burden of proof for any crime, including a criminal homicide, is Constitutionally on the State, not on the defendant. It's that Assumption of Innocence that liberals find to be so inconvenient when justly applied, particularly when that reasonable doubt comes into play.
For Murder in the First degree the State has to prove beyond reasonable doubt that the act was premeditated, planned in advance.