I am a lawyer in CT and practice in this area of the law.  The state's atty have what is called prosecutorial discretion.  This means that the state's atty can charge you with whatever they want.  State's atty's usually charge people with crimes that they believe they can make stick based on a reasonable finding of facts.  This difference between first and second degree murder is very serious and distint- i.e. one involvs premeditation and carries a more severe penalty.