I’ve been a juror in criminal cases in the past…
The biggest difference between murder and manslaughter is the intention in the mind of the killer. Murder is when the perpetrator successfully kills someone, whereas manslaughter is a terrible outcome of an unintentional action. (there is unintentional & intentional manslaughter, but that’s not part of the question posed)
First degree murder is premeditated.
Justifiability is the difference between criminal homicide and homicide (self-defense, etc.)