An unfortunate accident is about as useful in making Putin regret his was crimes, as is people saying they're a bit cross with Jimmy Saville for his dodgy behaviour, when in fact the latter died believing that he was the hero of the country.
Surely a suitable punishment would be to put Putin, and his sympathisers (especially his army generals) in rooms with the victims of bombs, rape, beatings etc, one at a time, and to listen to what they think of him/them. Not for him to get beaten up as we don't need to be as barbaric as this, but to have him forced to listen to what he's done to his victims, for hours, days and weeks without a break. And all whilst not being allowed out of that room at any time. The room, should be very cold, his only bed being a thin blanket on a hard floor, and with very little food. Altogether, something which forces him to think about what he's inflicted upon others. The punishment needs to fit the crime.
Whatever happens, Putin should not be permitted to die believing himself to be a hero.