Spoiler alert for Dr Who, Let's Kill Hitler and probably many things involving River Song before this point.
Let's Kill Hitler has come at last, and delivered a character focus that is uncommon Dr Who. Although there were politely murderous, betentacled metal robots, creepy robot clones of people filled with mini people and cupboard Hitlers, the real focus of this episode was on River Song and her relationship with the Doctor.
There are four major players in this episode:
River Song is the woman that Melody is not yet, but who The Doctor sees everytime he speaks to Melody. In this episode she exists only as The Doctor's conceptualisation of her. From this River/Melody emerges as a duality, and the conflict between The Doctor's conceptual River and the real Melody is what drives this episode forward.
Melody Pond is still a young woman, of sorts. She has just regenerated and she's found the man that she's spent her whole childhood obsessing about. She's been raised on a diet of The Doctor, The Doctor, The Doctor and by the time she meets him she's pretty set on his life belonging to her. When she hears him talking about this other woman it arouses her jealousy, hence the way that The Doctor's repeated mention of River's name never fails to slip under her attention. More than that, the warmth with which he talks about her provokes a desire to be that woman. Henceforth we get the duality and conflict between the two different forms of Melody and River Song, the conflict between the Id and Superego.
The Teselecta, are the simplest of the four major players. They tell you straight up what they are: justice. The Teselecta represent Melody's punishment for killing the Doctor. They represent the consequence of the inevitable criminal act. They are a literal articulation for both the fictional universes and the wider fanbase's reaction to the concept of Melody killing The Doctor. The Doctor however, puts himself between the backlash and Melody, pre-emptively forgiving her for the act. The Doctor is the one who prevents the revenge and tells us to forgive Melody rather than seek "justice". In many ways The Doctor is speaking as much to fans as anyone else.
So through relising that this woman he talks of, River, is her, Melody starts to become River. She's struck by the way The Doctor protects her despite her "killing" of him and through her own desire to become the woman that the Doctor speaks of. To reiterate an earlier point: the conceptual River (Superego) and Melody (Id) come into conflict and emerge as River Song (Ego).
When Melody first lands the kiss she comments that The Doctor is the master of all types of warfare "except for the most cruel". By the end of the episode River speculates that The Doctor must have known that she could save him, to which he replies: "Rule 1: The Doctor lies." The implication - a point that will no doubt remain ambiguous - is that The Doctor is far more adept at emotional warfare than Melody. After all, the episode ends with The Doctor having turned a deadly enemy, a weapon sculpted to take on the Doctor, and turned her into an ally.