Complex sentence: Clark assumes that she was killed in Doomsday's attack on the city, this and the death of Jimmy Olsen leads him to leave behind his human identity and fully embrace only his Kryptonian side.
Split into simpler sentences: Clark assumes that she was killed in Doomsday's attack on the city. This and the death of Jimmy Olsen leads him to leave behind his human identity and fully embrace only his Kryptonian side.