The ends do not justify the means.

I frequently tell people that the ends do not justify the means, and I mean it.

When I "cheat" on my diet, I am simply moving the goal posts (the ends) so that I don't have to "re-justify" the means.

I call this the Kobayashi-Maru of logic.

Please note that this is not a mixed metaphor, but a mixed idiom, or midiom.