Anyone else find the stuff about indifference to be rather deepish? Indifference has no power at all, it's the absence of power.

You're going to have to define "power" for that statement to be meaningful.

And now we learn that to cast Avada Kevada, you have to either want the person dead, or not care whether they live or die? So, I guess that means the only condition in which you can't cast is if you want the person to live, in which case you wouldn't want to cast it anyway.

I thought the point was that by default, a psychologically healthy human being assigns others' lives a value above zero, even strangers, and for Avada Kedavra you have to either overcome this with hatred (by assigning them a value below zero) or by indifference (by assigning them a value of zero). Both achieve the goal of expressing a preference for death (zero or below) over life (positive value), but the former requires an increasing amount of effort (because you're deliberately making yourself feel something) while the latter is "always on" and theferore doesn't.

I still don't see why repeat castings with hatred would require higher amounts of effort each time, but otherwise the concept is not incoherent.