What term would you use for a person who caused an incident
A neutral term for this would be principal:
[Merriam-Webster]
noun
1 : a person who has controlling authority or is in a leading position: such as
e : the person primarily or ultimately liable on a legal obligation
In other words:
Who was the principal of the incident.
Admittedly, it's not an entirely common use of the word, but it's still acceptable.
It could also be used adjectivally, a form that would be more common:
Who was principally involved in the incident?
This, too, uses neutral wording.