Let's preface by explaining that law in Louisiana is different than every other state in the United States. A student in a law school classroom will be told, "Here is the law for our State. Here is the law for most every other State. And then there's Louisiana."
The other 49 states in the United States use an English common law system derived from the British legal system and the Magna Carta. Louisiana uses a French civil law system based off the Napoleonic Code. (That's why Louisiana uses "parishes" instead of "counties.") While most criminal laws are specific to each state, there is something known as the "Model Penal Code" ("MPC") which is fairly universal. Louisiana has not adopted relevant provisions of the MPC as it relates to second-degree and first-degree murder.
Second degree murder can generally be defined as murder that is not committed in the first degree. That means to really understand second-degree murder, we must first look at what constitutes first degree murder in Louisiana. The first degree murder statute in Louisiana lists specific instances of murder that would qualify as murder in the first degree: murder committed in the act of enumerated inherently-dangerous felonies - also known as the Felony Murder Doctrine (§30(A)(1)), murder of a child or the elderly (§30(A)(5)), and when the murder is a "hit" where the perpetrator is paid to commit the murder (§30(A)(4)), to name a few.
To contrast, Louisiana's neighbor across the River, Mississippi, has a broad provision in the first degree murder statute that it derives from the MPC: first degree murder also includes any murder committed with premeditation and deliberation (Miss. Code § 97-3-19(1)(a-b)). "Premeditation" is a legal term of art; it does not require the mental images the work evokes based off movie and television. Premeditation does not require planning, and can occur in a matter of seconds. There are cases where premeditation can be found during the time between an assailant pulling out a gun and then pulling the trigger.
There is very little doubt a murder occurred, and there is no doubt as to who the victim(s) was/were, and little to no doubt about who the perpetrator is. The only issue then is whether the act would be considered first or second degree murder.
If Will Smith's death occurred in Mississippi, or almost any other State in the Union that has adopted the MPC, Cardell Hayes would likely be charged with first degree murder. In Louisiana, because the murder of Will Smith does not seem to blatantly fall into one of Louisiana's explicitly stated first-degree categories, all that is left is second-degree murder. Now, that is not to say Hayes will absolutely not be charged with first degree murder. As more facts continue to come out, and with the recognition that Will Smith's wife was also shot twice in the leg, there is chance this provision of Louisiana's first-degree statute could apply:
(3) When the offender has a specific intent to kill or to inflict great bodily harm upon more than one person.
"Specific intent" is another legal term of art that has a specific (no pun intended) legal meaning. The phrase can be substituted with words like "intentionally" or "knowingly," meaning the individual acts "knowing" certain results are likely possible outcomes. Whether Will Smith's murder and the injuries to his wife are two separate acts or one act depend on the locations of the parties involved at the time of the shooting. If facts come out to allege Hayes was shooting simultaneously at the bodies of two people in the front seat of the vehicle with the intent to kill or inflict great bodily harm on both, then it is possible the District Attorney could seek a first-degree murder charge. If, instead, facts come out showing Hayes intended to kill Will Smith and he did not even realize Smith's wife is also in the vehicle, then it cannot be determined Hayes had the "specific intent" to kill or injure more than one person.
Understand, though, that the consequential differences between first and second degree murder is paper thin. First degree murder brings about the possibility of the death sentence in the State of Louisiana (there has only been one execution in Louisiana in the past ten years). Both first and second degree murder allow for life in prison without the possibility of parol.
I'll be sticking around the comments section, so if you have any questions, just ask. As more relevant information surfaces that might change the legal status of the situation, I will make sure to keep everyone updated.