Scotty, it boils down to mens rea, the person's state of mind at the time of the crime. That's all the difference between reckless endangerment, manslaughter, and murder.
Reckless endangerment is driving with your cell phone on and running over a guy you didn't see because you weren't paying attention. You're still culpable because you shouldve been paying attention, but you had no intention to harm the man.
Manslaughter, in one form, is walking in on your wife in bed with another guy, grabbing a knife on the drawer, and stabbing the man or your wife to death. It gets knocked down to manslaughter because of a heat of the moment defense--your state of mind was clouded by your extreme emotional disturbance.
Murder is killing someone with premeditated intent. You knew a guy who was gay, and because you didn't like him, or for whatever reason, you plotted for however long to kill him. It's worse because you had time to think, so you shouldve changed your mind and let him go.