Lullabee
I have come to the conclusion that the Doctor is going just a teeny bit out of his mind. For most people, I suppose drowning a bunch of evil spiderbabies and not feeling bad about it isn't especially insane, but... those spiders were sentient creatures. And just children. They needed to be killed, otherwise they'd have gone and attacked London, but he just stood there like a statue, watching them die. He even seemed to be enjoying killing them, part of the time. And when Donna said to him about how scary he'd been, he didn't even seem to be upset then... and that's really scary. Yeah, when he was just standing there, soaking wet... his face was just kinda blank, just... watching... (sexy, but blank).
Well who can blame the guy, after all he's been though, I think it makes sense he's a bit cracked. Still... I agree with donna, he needs to find someone already. He needs a hug.