Role reversals and parallels - 1x01, 1x17, 2x04
In season one, Coulson firmly believes that taking a life is not an option, whereas May is the one who is willing to make the hard calls if the situation requires it. In season two, Coulson would rather die than be driven to madness the way Garrett was by GH-325, while May is the one fighting to make sure that doesn’t happen.
But some things never change. Coulson still believes in saving people and May is still focused on protecting him. It’s just that now, Coulson thinks that death may be the best way to save himself. And now, May is no longer going behind his back, but rather trusting him and his orders, which is why she ultimately respects his wishes and agrees to kill him as a last resort, even though it’s not something she ever wants to do.