Voluntary manslaughter is an intentional killing. The person facing the charge must not have had any prior intent to face voluntary manslaughter allegations. If the individual did plan to kill someone, then it's more likely that he or she will face murder charges.
Not planning to murder someone is the primary requirement of voluntary manslaughter. For example, if you're enjoying a walk with friends when you see your boyfriend with another woman, you might be enraged enough to attack him or her. If one of them ends up dead, then you can be charged with voluntary manslaughter due to attacking in the heat of the moment.
Killing in the heat of passion means that you killed as a result of a sudden quarrel or sudden event that enraged you. The term "heat of passion" can vary, but essentially, it means that it was a situation in which any reasonable person might have had the same emotional response knowing the same facts. It's an irresistible impulse, not premeditation.
It's understandable to get angry about a situation that is out of your control. The sudden surprise of a situation you weren't expecting could make you act irrationally and do something you later regret. For this reason, people who act out in a heat of passion are not typically charged with murder. It's simply not the same as planning to kill another person because of a dispute or long-time conflict.
If you are accused of murder, it's possible you could have the charge reduced to voluntary manslaughter in the right circumstances. Your attorney may have more information.
Source: FindLaw, "Voluntary Manslaughter Overview," accessed Oct. 04, 2017