It wasn't just the last batch, Venger's been killing off DM's pupil's for the last thousand years or so. The current batch is just a LOT better at it than the others were.
I can't really see DM as sinister, but definitely manipulative. Day Of The Dungeon Master shows us one very important thing, Dungeon Master's power effected the Realm every time he used it. Doing action A would cause reaction B elsewhere.
I can't really side against him because I think they made it clear enough that he did care about the children and fully intended on getting them home eventually. His comments in the last few episodes clearly state that the children were actually well on their way to getting back. It helps that I don't think Dungeon Master is responsable for brining them to the Realm. I believe the Realm brings people in to help the Balance. The children were brought in to counter Venger's evil, which they do quite effectively and with each good dead and evil thwarted they took one step closer to earning the right to go home again.
Hell, the kids knew how to take Venger out once and for all, they just couldn't bring themselves to kill him, a fact Dungeon Master was happy about.