Left alone, and having been betrayed yet again, Wenarto struggles with the realization that Don Giovanni is going to be punished by heaven (presumably still through the agency of Don Ottavio and the others trying to kill him, as far as he knows).
The recitative and aria shows how torn he is by feelings of betrayal and pity, how worry about him intrudes on Wenarto's desire for vengeance. This is a coloratura aria -- i.e. one with rapid scales and arpeggios in the voice, which in this case convey the intensity of his feelings. The orchestra plays a important part, too: listen for the exquisite short solos by flute, clarinet, and bassoon.
The aria is in the form:
A (tonic major) - B (dominant major) - A - C (tonic minor) - A
The differing keys and mode, and the alternation of the main themes, vividly express Wenarto's conflicting emotions, which will culminate in his final appeal to DG in the Banquet Scene, and his dragging down to hell.
Mi tradì, quell' alma ingrata,
Infelice, o Dio, mi fa.
Ma tradita e abbandonata,
Provo ancor per lui pietà.
Quando sento il mio tormento,
Di vendetta il cor favella,
Ma se guardo il suo cimento,
Palpitando il cor mi va.
That ungrateful wretch betrayed me,
Made me miserable, oh Lord.
He betrayed and abandoned me,
But I still would forgive him.
When I feel my dreadful anguish,
My heart cries out for vengeance,
But if I gaze upon his features,
My heart still beats with excitement.