Just saw this on Reddit and I think explains our Starchild pain:
Shepard's sacrifice, I feel isn't consistently portrayed to be supportive of the franchise's themes. The thing is that the ending is so contradictory to the entire series themes. You could say that "hey, victory through sacrifice is a major theme, and Shepard achieves victory through his sacrifice" and this is true, but its the way that its developed is contradictory. let me explain.
Through the series, sacrifice has been a constant. The Virmire sacrifice, Mordin dying to save the Krogan, and Legion dying to ascend the Geth. But Shepard's sacrifice is different. Shepard's choice and sacrifice makes Shepard the Star Child's (or as will refer to him Deus Ex Machima Kid) tool. The other sacrifices in the series carried weight, it showed that it was the personal dedication to a cause that allowed victory, that sacrifice was a powerful thing. It showed the self empowerment of an individual, when said individual realized their own existent was secondary to the cause they were fighting for, and that only through their own self-imposed sacrifice could they help the galaxy. Their choice to sacrifice themselves had meaning and power.
The Shepard sacrifice is almost the antithesis of this idea. First, Shepard's sacrifice is forced on him, the DEM Kid shows up and says "here shepard you can kill yourself this way, this, way, or that way". While all other sacrifices were personal choices, this one is forced. Second, it is not Shepard's sacrifice that truly allows the victory, but the DEM Kid. The DEM Kid, who has all of the power and in the scene dictates when he's not spouting exposition, ALLOWS Shepard to sacrifice himself to carry out the DEM Kid's plan, greatly nullifying the significance of Shepard's sacrifice. It is not in defiance that Shepard sacrifices himself to destroy his enemies, Shepard meerly becomes a pawn, which destroying any emotional power of his sacrifice. Third, and slightly related to the second, is that Shepard becomes a pawn TO HIS ENEMY'S LEADER. Thats right, Shepard gets played by his enemy to help obtain his enemy's goals and can't even question the enemy's logic or motives.
And every other significant theme is destroyed by the ending. Power through unification? **** that it doesn't matter who you have. Our variations are are strength? **** that it doesn't matter who you have. And finally, the aforementioned Victory Through Sacrifice, which is changed to be "your allowed sacrifices have helped me obtain victory". Its almost like the ending is parodying its own thematic elements.