I guess they needed yet another way to sell a newer toy of him. At first I (and some others) speculated that maybe it was an answer to Prime's alt. mode and that it would facilitate an awesome showdown between them. After seeing the movie, obviously this did not happen. Another wasted opportunity in DotM. Don't get me wrong, I liked DotM, it was the best of the 3 films, it's just that they could have used the truck mode better.
The way he was killed, I think the statement by Bay was clear: no more TF movies from me. (Or, at least, no more with Megatron in them.) He was thoroughly brutalized and killed. But, this is the TF Universe, nothing and no one stays dead forever.
And yeah, I'm still holding faint hope for a Leader Class version of Megatron. But it's fading by the day.