I actually thought Hitman was better than I expected. It was still bad, but it seemed a notch above other bad video game adaptations. Really, the only things that brought it down were logic flaws that you don't think about when playing the game... like why is it so hard to track down a tall bald, pale guy with a giant barcode on the back of his head? Plus, they tried shoe-horning in lots of ideas into the movie that people who never played the games wouldn't understand. But I think they did the best with what they had.
As for Halo, based on that live action thing they released, it looks like it would've been pretty horrible. I thought that video was some sort of youtube hoax, that's how bad it was. Like something you'd see on Sci-Fi Channel at 4 in the morning. I'm sure a Halo movie has the potential to be good just like the Halo games had potential, but in the end, I think it will just be padded with boring sequences, cheesy moments, and will just make you think about how much better movies like Aliens are. That's really all Halo is and all the movie would be... an Aliens rip-off.
From what I heard, I don't even think it would've focused on Master Chief, and that right there would've upset the fanboys. It's kind of funny, when you go to bungie.org and read about all of the connections and nuances of Bungie's games, you think wow they're either brilliant or incredibly lazy that they reuse these themes, ideas, weapons, characters, symbols, etc. Clever little easter eggs mean nothing without any meaning behind them.
Anyways, I think the biggest reason the Halo movie bombed is definitely because of Microsoft and their crazy demands.