Second degree murder is defined in California's penal code sec. 189 by what it is not and this seems to mean that it is not premeditated. Voluntary manslaughter is defined in California penal code sec. 192 as "the unlawful killing of a human being without malice, upon a sudden quarrel or heat of passion." Both sections can be found here.
I could spend the next two hours researching California case law to see if I can find the exact difference between those two statutes in California but I'll leave that to California lawyers. So you're going to get my very basic understanding of the difference between these two statutes: not a whole heck of a lot. But, you say "malice" - there must be malice in order to convict of second degree murder!
Sure there must. But malice in a second degree murder is begging the question. To prove malice you are pretty much going to have to prove "aforethought." Or, in more modern language, in order to prove malice you are going to have to show premeditative acts.
As an example, let's assume a set of circumstances.
Husband comes home and confronts wife while she is washing the dishes: "You whore, I know you've been sleeping with my boss!" They start screaming at each other and she takes the knife she was drying and stabs him in the heart.If she did this out of pure animal anger, with nary a thought until she is calling 911 in a panic, this is voluntary manslaughter. If she thinks to herself "I've got to do something. This can't get out." and then plunges the knife into his heart, that's first degree murder. At least it is if California follows the rule that premeditation can occur the second before the murder (which I understand to be the law in most places).
So what is 2d degree murder? Technically it's malicious homicide without forethought. If you cannot prove forethought through some sort of action how do you prove malice? If you convince people that malice exists they are going to be convinced that a second of forethought exists (if nothing more than "I'm going to kill you, you son of a buck!"). With that in mind, 2d degree murder becomes an option to offer a jury (or judge) which isn't all that comfortable with convicting someone of a crime which we all know would subject him to death or life in prison.
So basically, 2d degree is homicide without real proof of actions indicating a malicious intent to commit murder. This is pretty much the same evidence which would support voluntary manslaughter. Again, how do you not give these instructions simultaneously?