I think it was the delayed shock of their realising that they'd had their brains cut out and put into metal behemoths, and possibly the incredibly hideous feeling of having a hideous metal body thing. I'm not so sure about the exploding heads, though - I reckon that was a suicide, which is probably much easier if you're made of wires and things.

And of course it was cruel and unusual - did you notice the reference to Genesis of the Daleks in there? When Four had the chance to wipe out the Daleks, he said "Have I the right?", and put the wires down and didn't do it - Ten asks "Can I do that?", which is quite different, and then does it anyway, while manipulating Mickey remotely. More fuel for my theory that 10=7, dammit.