Answer:
The main difference between first-degree murder and second-degree murder lies in the previous planification of the murder, made by the person who commits the crime.
In first-degree murder, the murderer plans the crime: he is aware of what he is going to do, and plans the details of how and when to carry out the homicide.
On the other hand, in second-degree murder, although there is also intention and discernment regarding the crime that is being committed, it is not a planned act: it is a voluntary action, but not premeditated.