"Formal Conflict"... You're not allowed to kill the opposing Nobles/Officers. Instead you take them hostage for ransom.
Commoner soldiers/levies and mercenaries under contract do
NOT fall under this rule, and as such don't count...
Of course, Accidents Happen in wars, and it wouldn't be the first time such Accidents cost the life of a Noble of Inconvenient Lineage, especially in territory/inheritance related scuffles like this..
Especially when the gain is well more than the Blood Money owed to the family of the sadly expired noble...