Should has several uses, not all of them strongly related. This is a very specific one. Maybe André is from the US, I think this construction is largely used in the UK, consisting of: main clause expressing an opinion + 'that' + subordinate clause with 'should'. "I was surprised that you should treat me so." simply expresses my surprise at how you treat(ed) me, I could have used 'treat', 'could treat' or 'treated' instead.

I think should is incorrect ...the correct word in this context is "would" predictable behavior .

Have you studied a construction known as the subjunctive? If you haven't, check it out in your nearest grammar book, and you'll see that this construction is a kind of subjunctive. Here are some more examples: It's interesting that you should say that. He insisted that I should go. The modal verb 'should' creates a kind of 'distancing' between the main clause and the subordinate one. In most European languages, you would have to use a subjunctive mood in the second clause. English, by contrast, avoids the subjunctive in all but the most formal of contexts. One of the ways we avoid it is by using the modal 'should' ( a relic of our Germanic roots). This is more common in British English than in American English.