Please refer to the previous discussion on should:
I would choose (a), which suggests a surprising fact to the speaker/writer that he committed the crime.
(b) is possible too, as far as I know:
I was surprised that he should sound so definite: it was usually I who pinned down occasions with that sort of fact. (Quoted from A Woman of My Age).
I wonder if there is any further difference between (a) and (b) besides what was suggested in the previous discussion by Yun that in (a) committing the crime happens before the speaker's or writer's feeling surprised. According to Yun's interpretation, (b) suggests both the verbal actions happen in the past: does this mean the two actions happen at the same time? If not, why the different tenses as in (a) and (b)?