Archie MacMillan was Evan MacMillan's father and greatest idol, since Evan admired the way his father ran the MacMillan Family Estate.
Archie was first and foremost a businessman and held a tight ship with strict attitude towards the servants and staff at MacMillan Estate, often working them long and hard for little pay. It was this attitude that Evan idolised.
Years later, Archie's mental health faded away and a once loyal and neighbourly man became a near-bedridden, frail old man. Evan spent a fortune on ointments trying to cure his father, but nothing seemed to help. Over time, Evan became fed up with his father and applied the same icy logic as his father had applied to the servants and staff: those who cannot carry their own weight do not deserve the weight they carry.
Archie eventually disappeared from the outside world and it is assumed that Evan, who later went on to claim responsibility for the worst mass murder in modern history, had killed his own father.
In Year 3, an update to the game's Lore shifted the main operator in the MacMillan Estate disaster from being Evan MacMillan to his father Archie, thus also shifting the Players' perception of Archie MacMillan as a person.
Whereas he used to be a generally friendly and hospitable man, but a strict businessman at heart, the new Lore revealed Archie to be a cruel, evil man before his mental health deteriorated and his son Evan took over.