The difference, in a word, is “intent,” but like everything else in the law, there are exceptions. To be guilty of simple murder you must intend to cause the death of a person and perform an act that results in death. For example, you scream, “I hate you John!” and shoot him between the eyes with a handgun. As crimes go, simple murder is pretty straightforward, with one exception. Generally, an accident can’t be considered murder. But let’s say you shoot at John’s head, trying to kill him, but the bullet misses and hits and kills Mary. Mary’s death is an accident, but it is still murder because of the doctrine of transferred intent. The law takes your intent to kill John and applies it to Mary, turning her accidental death into a murder.
There is another exception to the intent requirement for murder, felony murder. The specifics will vary from state to state, but basically, if you are committing a serious crime such as robbery, kidnapping or sexual assault, and an innocent person gets killed, you can be charged with murder even though the death was unintentional. Moreover, if you have one or more partners in crime, and a partner causes a death, you can be charged with felony murder even if it’s not your fault, unless you fall within a complicated exception. Felony murder only applies if an innocent person is killed. If a partner in crime is killed, you can’t be charged with murder unless you intentionally killed him or her.
Manslaughter most often applies to accidents that result in death, but again there are exceptions, and the laws vary from state to state. Generally, the highest degree of manslaughter requires conduct that is unintentional but so reckless that it created an extremely high risk of death to another person. An example is two people deciding to play “Russian Roulette.” Each loads one bullet into a revolver. They take turns spinning the chamber and shooting at each other’s heads. If someone dies, the one left alive can be charged with the highest degree of manslaughter. Lesser degrees of manslaughter apply to people who recklessly cause death in less obviously dangerous ways, such as driving drunk, leaving loaded guns around where kids can find them, or driving at such a speed or in such a manner as to seriously endanger others.
Just as there are exceptions to the “intent” requirement for murder, there are exceptions to the “accident” requirement for manslaughter. Again, the specifics will vary from state to state. If you intend to seriously injure but not kill someone, and they die, you can be charged with manslaughter. An example would be sexually assaulting someone so violently that they die, or beating someone so badly that it kills them, even if you didn’t mean to kill them.
Another exception is that an intentional killing committed under the influence of extreme emotional disturbance can be considered manslaughter instead of murder. The law school example is a loving husband coming home after a long day, finding his wife having sex with another man in his bed, and shooting her. Or a wife killing her husband moments after her young daughter tells mom that daddy has been having sex with her. The thinking is that people acting in the heat of passion right after experiencing extreme emotional trauma should not be held as accountable for their actions as others. Thus, extreme emotional disturbance converts an intentional murder to what is called voluntary manslaughter.
Another exception that might exist in your state is assisted suicide. Helping another to end their life obviously requires the intent to cause a death. Most states have chosen to punish assisted suicide as manslaughter, or have made it a separate crime, so that it can’t be punished as murder. A few states have decriminalized assisted suicide altogether.