It's not one or the other.

Force majeur describes a disaster that no one could have predicted or prevented. This does not apply here since the god of storm could have prevented the disaster by choosing not to rain down lightning bolts. They are the God of Lightning after all.

The God of Lightning chose to kill those people and then did. That is called murder, and by human laws, they should be locked up in some sort of rubber cage. Perhaps we can attach an electrode to one corner of the cage and get free renewable energy.

How much responsibility falls on you depends on how often the God of Lightning has done this sort of thing in the past. If they have never done it before then it should be treated as unpredictable from your point of view, and you are not to blame. For example if the god has full control over their powers and has never killed anyone before.

Imagine instead of the god of thunder you invited former US President Barry Bar-Bar-Obama to open the event. But midway through his speech he starts chucking hand grenades into the crowd. You will not be blamed for this (provided the grenades were well hidden).

The other option is the god has no control of their powers. They are followed everywhere they go by a storm. Sometimes the storm gets much worse and they don't get to decide when. This is similar to if you invited the Joker from Batman to open the event. Everyone knows he's dangerous after all. So you endanger everyone at the event by bringing him here.

Of course it is not a case the responsibility is split evenly between you and the guest. The speaker is treated the same in all scenarios. The background determines how you are treated.

It's not one or the other.

Force Majeur is a disaster that no one could have predicted or prevented. This does not apply here since the god of storm could have prevented the disaster by choosing not to rain down lightning bolts. They are the God of Lightning after all.

The God of Lightning chose to kill those people and then did. That is called murder, and by human laws, they should be locked up in some sort of rubber cage. Perhaps we can attach an electrode to one corner of the cage and get free renewable energy.

How much responsibility falls on you depends on how often the God of Lightning has done this sort of thing in the past. If they have never done it before then it should be treated as unpredictable from your point of view, and you are not to blame. For example if the god has full control over their powers and has never killed anyone before.

Imagine instead of the god of thunder you invited former US President Barry Bar-Bar-Obama to open the event. But midway through his speech he starts chucking hand grenades into the crowd. You will not be blamed for this (provided the grenades were well hidden).

The other option is the god has no control of their powers. They are followed everywhere they go by a storm. Sometimes the storm gets much worse and they don't get to decide when. This is similar to if you invited the Joker from Batman to open the event. Everyone knows he's dangerous after all. So you endanger everyone at the event by bringing him here.

Of course it is not a case there is a given amount of responsibility which is split between you and the guest. For example there is 50 years imprisonment for each person killed, and that must be divided among you and the guest. That is not how it works. It's more like 50 years per victim goes to the guest, and then the background will determine how many years you get in addition.