There are probably lots of pivotal moments, but none of them are the pivotal moment that would allow Order to overcome Chaos.
Save a planet or two for a few more centuries? Sure, yeah, that sort of thing turns on the smallest things.
Save millions or billions of lives? Yeah, that works. It's a drop in the bucket, but it works.
Prevent the Horus Heresy? Probably not doable, because the powers of the Warp already used time travel to help the Heresy happen in the first place so there's some closed loop stuff already going on.
Reverse the inevitable slide of mankind into depraved and torturous hell? Not possible, because mankind is already in it, and has always been in it from the moment our species first attained sapience.
I would say the turning point was long before that even. There's a lot of really horrifying technology that came out of the peak of human civilisation, and they seem to point to humanity still using slave soldiers, engaging in blood sports, using cyborg-servants and so on even when they were capable of building fully sapient machines.* They tortured and mutilated and mind-controlled not because they needed to, but because they wanted to. Because humans in 40k have always wanted to.
*Setting aside the moral concerns with replacing human slaves with robot slaves with the same or even greater level of intelligence.