Homicide is manslaughter in the first degree in the following cases:
1. When perpetrated without a design to effect death by a person while engaged in the commission of a misdemeanor.
2. When perpetrated without a design to effect death, and in a heat of passion, but in a cruel and unusual manner, or by means of a dangerous weapon; unless it is committed under such circumstances as constitute excusable or justifiable homicide.
3. When perpetrated unnecessarily either while resisting an attempt by the person killed to commit a crime, or after such attempt shall have failed.
R.L.1910, § 2320.