Why didn't Ralph just plead the 5th when he was on the stand?
Seems easier than admitting you did some nasty things to some boys and being a grown man crying...
It wouldn't have mattered too much to the story if he had done so, his testimony was to be disregarded by the jury in the end. But we as the audience may have felt disappointed by the lack of rape accountability. Seemed a little cheap.
The general consensus is that, believe it or not, he actually felt bad about what he and the other guards had done, had tried to forget about it just like the protagonists had, but now that he was tricked/coerced into admitting it he actually was glad to unload it all.
What bothers me most is that Michael had no way of knowing that Ferguson would confess on the stand; just because he wasn't a criminal like the other two living guards didn't mean necessarily that he had any remorse, and yet Michael was hinging on this in order to make the truth about the reform school publicly known.
Ralph was taken by surprise and most likely assumed from the nature of the questions that the defense already knew what he and Nokes had done, and that it was going to come out regardless. Obviously he could've taken the 5th but how would it look if he refused to answer whether he raped kids in the past? Plus it did seem that he felt bad about what had happened.