The whole idea (spoilers)

Look, I'll preface this by saying it was a crappy movie with crappy acting.

That said, it dawned on me what I think the father's ultimate motivation was. There are several posts where people argue that only a few of the people belong in the room, and I myself wondered why he didn't shoot the ADA first. (except for the fact that she was needed for the plot point)

However, I think the REAL reason the father did this was to expose the district attorney as the murderer and liar she is. He mentioned how he was always in control at the end, and what I think he meant.

The other people there were necessary to get her to expose herself, and most of them aren't exactly innocent pawns, either. I'm sure he also wanted to kill the man who raped his son and gave him AIDS - but I think when he investigated be saw how horrible and was and that SHE was the real reason his innocent son went to prison.

Even if he exposed her crime of forging evidence he wouldn't be able to prove she was the murderer he saw her as - unless he put her in a situation that forced it out of her.

Maybe I'm reading too far into a crappy movie, but it certainly answers most of tuff questions floating around here.