Answer: Voluntary manslaughter
Explanation: Since the man found his wife with a neighbor during sex, not knowing beforehand that his wife was having an affair, it excludes the possibility that the man had planned the homicide beforehand. Another option is the unintentional, or accidental murder, so called involuntary manslaughter, where it would occur, for example, if the gun were fired accidentally, without the man's intention to kill the neighbor. This option is also ruled out because the man shot his neighbor with the intent to kill him.
So in this case, no murder was planned in advance, but upon seeing his wife's situation and affair with a neighbor, the man decided to kill the neighbor on the spot, so this is a voluntary manslaughter with murder.