I wanted clarification on what techniques are employed in the following quote:
I was benevolent and good but misery made me a fiend.
Is this an example of high modality, antithesis - or is there something more specific?
This is a pretty classic example of antithesis:
a figure of speech in which an opposition or contrast of ideas is expressed by parallelism of words that are the opposites of, or strongly contrasted with, each other, such as “hatred stirs up strife, but love covers all" [NOAD]
There is additionally a slight chiastic structure, a reversal of order: "I was X ... something made me Y .."
As well, there is some hint of prosody and symmetry: The first clause has four strong beats, and so does the second.
And, finally, there is some alliteration: "misery made me" ...