You are one of the few people in the house who had an access to your father's private office. Today, you have noticed that an antique gun has gone missing from the study - an old antique that, despite its decent age, was in working order. Most likely, that Henry was killed with this weapon. Jean-Jacques had always been a desperate collector of rare things - he could have easily killed ypur father for the antique weapon.
You don't suspect Letizia - your wife was there all night. At least you fell asleep together...
You've looked at Eric's gun collection many times. Your father's primitive nature has always fascinated you. It's scary to imagine what could have happened if it had broken through.