Various sources plus an achievement ("experience all four endings") in Secret Files 3 after finishing the game state that there are four possible endings.
Which actions decide the ending?
One point might be this one, but even though it's rather late in the game I'd prefer to have a confirmation than playing it again from that point just to see the same ending as before.
Saving Max vs not saving Max after he was poisoned. He survives in both cases but might be pissed in the latter case.