The Red Viper isn't one of my favourites, but this is definitely one of my favourite images of his armour (and Dornish armour in general); I also find his 'Evil Spock' goatee and smirk entirely appropriate.
I would never deny Prince Oberyn's devotion to his sister's memory, but I can only consider his failure to finish off The Mountain that Rides while he had the brute down and at his mercy to be the least laudable manifestation of his fraternal piety - it was both sadistic and stupid, FATALLY stupid as it turned out.
If you want a dangerous man dead KILL HIM and don't try to finish the job by TALKING him to death.
Let me put it this way, I find it easy to admire The Red Viper's cunning and showmanship (even if he died, his poisons assured the death of Ser Gregor too), he shows a certain lack of common sense in being willing to risk his neck by placing it in close proximity to The Mountain's still-mighty paws!
I must admit that his desire to hear Ser Gregor confess seems just a bit too much like hubris for comfort; it's as though he wanted to have his cake and eat it WITHOUT taking a photograph - one almost has to wonder if Prince Oberyn was a little surprised by just how well he was doing in this particular single combat and let the ensuing case of Victory Fever go straight to his head! (understandable, but still surprising in a practiced killer - another sign of his love for his sister and rage at her murderers, perhaps?).
I quite agree, but must still suspect that Prince Oberyn was getting just a bit TOO ambitious for his own good! (much as I'd like to see Lord Tywin brought to account, I doubt it could be done in the course of a single duel).