I’m not sure this is correct - page 21 of rule book re: attacking multiple targets says “each of these attacks is a separate action”, not a single action.
Reactions interrupt flow of play and page 24 says that once a reaction is resolved, play continues from the point where it was interrupted, unless the reaction has made that impossible. In this case, Severin’s reaction killing the attacker would make it impossible to continue.
But simple way to avoid this issue for the attacking player is to do the attacks against Severin last (attacker chooses order as per page 21).
just my thoughts ?