I'm not 100% sure for this, but you can't subtract this way in ECC.
Think of this. 1*G - 1*G would be equal with 1*G + (-1)*G, right? But, -1 is outside the range. In finite fields, -1 is equal with N-1 which is 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364139.
So, what you really do is 1*G + (N-1)*G which is N*G. Apparently, k*G + (-k)*G is always N*G, but I need this to be confirmed. Anyway, check:
How to subtract two points on an elliptic curve?Subtracting a point by itself always leads to zero because one of the points is at the top of the Y axis, the other is on the bottom of the Y axis but has the same X coordinate so they just have a flipped Y coordinate.
Since subtraction is just negative addition, and point addition just finds the third point on the curve that forms a line with the other two points, when you're subtracting a point from itself that third, intermediate point does not exist so it passes through the infinity point that's in three-dimensional space, which is how you arrive at the 0G result. It would not be equal to -1G.