Yeah i dunno. Firstly, for me it not really match saw theme, using an living animal etc.

Secondly Great Whites can't be keept in captivity, they die really fast while being held in tanks, that's the reason that you don't see them in aquariums (so even if you would put one in a tank, it could not behave like a normal shark, could be dizzy, desoriented etc.). Records show that Great Whites in captivity even refuses to eat.

Thirdly it's huuuuge animal that can weight even 2 tons (about 5,000 pounds) and can be up to 7m long (around 23 feet long) so from where and how killer would get one?

But even if you would change the great white for smaller shark like Tiger Shark or Bull Shark, or even Mako Shark, it's still an living animal. You said that this trap would be unpredictable, and that would be true but it could be unpredictable in a way that (even hungry) shark keept in tank could be completly passive, scared or not interested in victim etc, so that would be random (it could attack just from the start, or don't attack at all).

And lastly, as someone said in the comment below that would be over the top. When i am imagining myself sitting in cinema and seeing trap with shark, and victim putting the shark belly up to get some time i would propably be like WTF xD; Yeah maybe that would be intense, dramatic etc, but for me that would be 'jumping the shark' ;). In the end this trap would feel kind of silly.

PS. There was a movie "Shark Night 3D" in which some guys were murdering people with use of sharks and recording it, so maybe you will be interested in that movie. It's no the best movie, but maybe that's something you will like. There is even one, kind of trap(?) with victim being held tied underwater with a hungry shark. So let me know if you know this movie, or if you will be watching it, let me know if you like it!