Why does this graph show the tightness of MST heuristic's 2-approximation bound?
There is a lot of freedom in the algorithm: several minimum spanning trees, and several corresponding Eulerian tours. Try to find the worst choice and show that it produces an inferior tour. What you are showing is that the algorithm could make this choice, and so could produce an inferior approximation.

If you don't like this idea of choice, you can sometimes guarantee that the algorithm makes the choices you want by slightly tweaking the weights.