france
Criminal liability does not care much about "lesser victims"
US and French penal sentencing differs significantly on three aspects:
- concurrence of jail terms
- standardization of sentencing guidelines
- plea deals
The differences are detailed below. Because of those differences, deciding to charge (or not charge) attempted manslaughter/murder in French courts is mostly about having a "fallback" in case the more serious charges are beaten (which would be difficult under the given fact pattern), and will not significantly move the sentence.
French jail terms are concurrent
Article 132-3 of the penal code:
Lorsque, à l'occasion d'une même procédure, la personne poursuivie est reconnue coupable de plusieurs infractions en concours, chacune des peines encourues peut être prononcée. Toutefois, lorsque plusieurs peines de même nature sont encourues, il ne peut être prononcé qu'une seule peine de cette nature dans la limite du maximum légal le plus élevé.
When, within a given procedure, the prosecuted person is found guilty of multiple related infractions, every possible penalty may be imposed. However, when multiple penalties of the same nature (*) are possible, only the highest possible penalty may be imposed.
In US parlance: every period of jail time imposed within the same trial is run concurrently.
Therefore, if you are found guilty of murder, the maximum sentence does not depend on attempted manslaughter attempts in the same series of facts. The actual sentence will depend on it, but it does not require charging the manslaughter attempts.
(*) Penalties are mostly jail time and fines, but specific infractions give rise to a fairly diversified set of "peines complémentaires": license revokation for drunk driving, ineligibility for corruption by public figures, etc.
Standardization of sentences
Furthermore, the US sentencing culture is heavily influenced by the federal sentencing guidelines. They might be non-binding on state courts, and can be deviated from in federal courts, but they still tend to encourage a "mathematical" view of sentencing (if guilty, then go through criteria A to Z, calculate a "score" and sentence accordingly). This has advantages (in particular, the USSG reduced a previously enormous race bias) but it also reduces the personalization of sentences (which is necessary for other reasons).
In French courts, there is no such standardization. Common infractions will often have a "tag price", but those "tag prices" might differ across courts, and can be "haggled with" considerably more than in the US. Mass murder is not a common infraction and will be sentenced with considerable individualization.
"Plea deals" do not exist for felonies
Given the above factors, a French prosecutor has much less control over the sentence than her US counterpart.
There was a considerable shift towards plea-deal-type procedures in French criminal law in the last ~20 years. I am not familiar with the various procedures but a key point of all of them is that (A) the prosecutor still charges for the largest offense, (B) the judge can reject the deal with much more discretion than they would have in the US.
This shift was mostly oriented towards mass infractions, where the "tag price" was already established due to a large market of sellers (offenders) and buyers (law enforcement catching them). Typically, those would be small-time drug dealing, petty thievery, and driving offenses. The policy goal was (obviously) to reduce the costs associated with the justice system.
Felony cases are (thankfully) rare and involve considerable risk for the defendant. Plea-deal procedures do not apply to them.
Civil liability and procedure
Civil liability
On top of criminal charges, could there also be a lawsuit for pain and emotional suffering by the attendees
A very rough summary is that French law, both civil and criminal, quantifies "pain and emotional suffering" through the incapacité temporaire / totale de travail.
Basically, non-tangible harm is quantified by medical certificates establishing that the affected person could not "work" for a given duration. "Work" is somewhat misleading though; see the governmental guidelines for medical practitioners:
l’incapacité ne concerne pas le travail au sens habituel du mot, mais la durée de la gêne notable dans les activités quotidiennes et usuelles de la victime (...). La période pendant laquelle une personne est notablement gênée pour se livrer à certaines des activités précitées est une période d’incapacité (...)
the inability does not refer to work in a professional context, but to the duration of a non-trivial impediment for the victim to pursue its daily routine activities [eat, cook, buy groceries, etc.]. (...) The duration of significant impediment to such activities counts as incapacité temporaire de travail.
It is very likely that people in the crowd, even if not hit by bullets, would exhibit some trauma symptoms giving rise to temporary and/or durable "impediment to daily, routine activities". This might also be the case for close family members of those in attendance, etc.
Class action (almost) does not exist in France
(maybe a class action to simplify the proceedings)?
Technically, class actions are possible under French law since 2014. However, they are extremely rare (this 2024 source counts 32 procedures in total, 6 of which did not end in a dismissal). They are also restricted to consumer law violations; a terrorist attack would not do.
That being said, "standard" lawsuits that look a lot like class actions do exist. It is not uncommon for a large number of plaintiffs with similar claims to be represented by a single lawyer (or team of lawyers). If a court in such a case suspects that various groups of plaintiffs have diverging interests, or that individualized factor will matter, they will most likely remind the lawyer(s) of their obligation about conflicts of interests. This would be class certification in all but name.
Furthermore, (part of) the public policy behind class actions is to discourage bad behavior that causes small damage to numerous persons. French law has a different approach to that problem, by giving standing for certain moral persons to sue. Hence, environmental NGOs have standing to sue in administrative court to cancel building permits on undevelopped land, consumer groups have standing to sue in civil courts if a defective product was found on the market, etc. Such moral persons are entitled to damages even in the absence of individualized injuries, and they do not have to redistribute the proceeds to injured parties.
(I do not have the specific knowledge to comment on the exact economics of such cases, nor do I care much to investigate, given that this is not central to the question asked.)