I thought that all along, as a strong possibility anyway. However, if they were all brutally killed then I may rethink..And also since the victims, besides the owner, were employed in the maintenance area of the business, there may be a reason for all of them.
One more thing, maybe none of them was specifically targeted and he just showed up to kill whoever was there, to make a point (whatever?) Otherwise, wondering how he would know those 4 exact people including the owner would be there that early on a Monday morning. Did he have and appt? JMO