In the first two films the Decepticons were hellbent on annihilating all or at least most of the human race. TFilm had them going after the AllSpark for its ultimate power; although Megatron offered to let Sam live to be his pet, he’s obviously going to finish off his pets once they no longer entertain him or serve further purpose for him. In ROTF it was pretty much the same deal: they were looking for a means to activate the solar harvester machine, which would not only give them enough energon to keep going but would kill off the human race. DOTM took a somewhat different turn: the Decepticons maintained a spy ring of humans to do their work, and with the Autobots out of the way, take over Earth and make the humans their slaves. This change was caused but by the need for labour to rebuild Cybertron. They possibly saw how useful human slaves are, but it’s more the fact that they just like the idea of having slaves to control and that it’s easier to use foreign labour than their own for rebuilding their homeworld. On the whole, I thought DOTM’s plan was rather impractical: - First off, how long would it take humans to construct an alien machine empire? Besides, how were they going to rebuild Cybertron without the AllSpark? - Secondly, Decepticons are incapable of looking after slaves; they’ll probably work them all to death or even massacre in the first decade of slavery or so. I don’t know if Sentinel was insane enough to use human slaves to rebuild Cybertron, or whether he just entertained the idea of slaves to rule over. Either way, human slaves wouldn’t have lasted long. - Thirdly and most obviously from these films, humanity has proven themselves as worthy foes, twice over. Taking such foes as slaves may sound great, but it puts them in a position where they can turn on you easily. It would have resulted in a man vs machine war worthy of THE TERMINATOR series. As seen in the film, NEST Demolition team did some serious damage to the Decepticons and an aerial strike took out most of the gunships, so it was quite feasible that humans can rise against the Decepticons. Slavery is arguably worse than death: it’s a slow, degrading and painful death. But still I thought the whole “kill them all” thing the Decepticons had going for them was more awesome and menacing (Blackout, Frenzy, Ravage, Laserbeak). After all: only trophies, no prisoners or even slaves!