The thing about using it to crash and destroy a city is a bit unbelievable, isn't it? I mean, the moment that plane is detected it'll be shot to smithereens. I cannot see it being a more effective way of terrorism than a repeat of 9/11 - I know it would be harder these days, but not as hard as this whole thing, surely?!
I just can't see it.
Hostages? - well, it's been a week now and no word, unless the passengers managed to crash land.
Simply crashing - then why all the course changes, switching things off etc?
If I had to come up with a best match theory, I'd say someone controlled that plane remotely, turning stuff off, going this way and that, up and down, it's the kind if thing you'd do if you were testing out a remote control. And then they ditched it in the sea.
Nothing else sounds completely plausible to me.