Should Johnny & Ponyboy be charged with murder or manslaughter for Bob's death? Why?

Under United States law, murder is defined as:

the unlawful killing of a human being with malice aforethought (The US Code, at Title 18)

This definition implies that the person or persons charged with the act had to have entered into the commission of the crime with the intention of doing harm. Murder is intentional.

Under United States law, manslaughter is defined as:

the unlawful killing of another, without malice, either express or implied; which may be either voluntary, upon sudden heat, or involuntary, but in the commission of some unlawful act (The US Code at Title 18, §1112)

In light of the circumstances, the logical charge would be manslaughter, but as has already been indicated, the likely outcome would be a defense of self defense.