To save & keep the Continental
Winston makes it very clear that he effectively betrays John Wick because the High Table will otherwise kill him.
He makes the deal to keep the Continental and run it as he wishes.
However, regarding that latter one and the following battle at the Continental, Winston declared that this had all been a show of strength, indicating that he wasn’t so much interested in helping John, but wanted to show The High Table he’s still not to be trifled with.
Even if Winston is more fond of John Wick than most people in this colorful underworld, he might not be willing to risk this friendship if it means losing The Continental. So when it came down to choosing one or the other, he chose keeping control of his hotel, and that meant John had to go.
Of course, it could all be a ruse but we'll have to wait for Chapter 4 to find out.