The original Destro's fate is pretty horrific. The French essentially heated two halves of a metal mask until they were red-hot, then jammed them onto McCullen's face, complete with the audible sizzling of burning flesh.
The Nanomites Cyanide Pill, which (very graphically) eats away the afflicted Neo-Viper's body in under half a minute, rapidly stripping away his skin to reveal the muscle and then the bone beneath, until he just dissolves into greyish dust filling the remains of his suit.
Similarly, Destro's transformation. Aside from some pretty horrid-looking facial burns, we're treated to the sight of metal visibly growing out through the burned flesh until it covers the entirety of his head.
The Eiffel Tower collapsing.
The flashback showing Rex attacking his own sister in her sleep and injecting her with brain-washing nannies.
