Teenager
The 15-year old child of one of the victims, here as part of a "take your daughter to work day". The parent was probably a somewhat incidental victim.
Seamus almost certainly didn't realize that there would be a kid present, and was probably horrified when he saw him or her.
Maybe the parent was an accidental victim? It would be horrible if the teen tried to attack, pushed his arm, and the Sureshot produced its usual perfect kill.
Or, could be child of the CEO. This would work well with the wife. Could do both.
