He was loyal to The Partners up until they made it clear that his past loyalty meant nothing to them, and they insisted he inject the killswitch into his neck. He’d just seen how callously they disregarded his mentor, and they’d just put his life in the hands of those he considered subordinates (the Washington twins).
The implication being that he knew about The Partners “secret” plans all along, and actively sabotaged them in anticipation that Diana and 47 would either find a way to discover The Partners with or without his help.
It’s kind of interesting in that they subverted the traditional buid up with this sort of plot structure, usually the Constant would be introduced as the main villain and the reveal of The Partners as his puppeteers would be the escalation of the threat. In this case they used the Constant turning on the Partners as the escalation since now he had both the smarts and the funds.
Based on interviews it seems that IO Interactive had several plans going in and kept developing and redeveloping them as things went on, which is honestly a wise idea with a project of this scale where you’re releasing in parts and then getting feedback in response to each part.
However the Constant was introduced as the “face” of Providence early in Hitman 2016, so it seems likely he was always going to be the main antagonist. What’s less clear is how they wanted to characterise him, if they wanted to do more juxtaposing him with Lucas Grey, etc.