 If the tangent line at the right end point is considered (which can be estimated using Euler's Method), it has the opposite problem

The points along the tangent line of the left end point have vertical coordinates which all underestimate those that lie on the solution curve, including the right end point of the interval under consideration.The solution is to make the slope greater by some amount.Heun's Method considers the tangent lines to the solution curve at "both" ends of the interval, one which "overestimates", and one which "underestimates" the ideal vertical coordinates.