Prantagonist
A character of a story who is the protagonist who, through a series of plot twists, ends up to also be the antagonist
a combination of protagonist and antagonist
a combination of protagonist and antagonist
Alex Mercer from Prototype could be considered an prantagonist
Prantagonist
When the main characters are both good, but oppose each other.
IN Batman v.s Superman, they were both prantagonists.