Its also been done in the X-Statix comic with the Bush Rangers:
Criminals experimented on and the few survivors mutated into super monster criminals.
The concept of experimenting on prisoners is his origin. So either they use it and keep
it similar to the comic or they change it and make it different and perhaps upset some
fans. But the idea of experimenting on criminals with the super soldier serum could
easily be explained. In the comics, Ultimates at least, SHIELD had experimented on
several test groups of SHIELD agents and all of them died, group after group of like 20
men, all dead because of the serum. Banner (in the Ultimates, which it seems
Avengers is going with) accidentally became the Hulk because of a similar Super
Soldier experiment. But they could easily explain that they didn't want to continue to
kill soldiers so they started experimenting on criminals. And it can be shown that they
took every foreseeable precaution in case he did become strong and powerful. With
guards, locking his amrs and legs down, gas, etc. Just like the cell they created for
Loki. But when the experiment went wrong those precautions didn't help.
If its Hydra or AIM doing it it could be explained as they don't care about the morals,
and they can't do the experiments legally so they took over a prison for the test
subjects, who they would kill anyway even if it worked.
If it was SHIELD, they maybe picked men from Death row who they knew were good
men, maybe men who they could tell would be good guys but were convicted and this
was their only way out or something.
I don't know, I am not writing a whole script here. Just coming up with some ideas.
The results wouldn't be the exact results that happened to Captain America, Cage
wouldn't increase in mass like Steve, and he wouldn't have developed anything but
strength and bullet proof skin. No increased agility, speed, increased brain power
(which I think Cap has that allows him to think faster), and no military training.