After his daughter Amber died, Dr. Fuji devoted his life to trying to bring her back through the usage of cloning. His wife was distraught and could not continue watching his endeavors, insisting that he could not bring Amber back to life. She eventually left him, yet Fuji still worked to find a way to clone Amber.
Eventually, his studies gained the attention of Giovanni, who said he would fund Fuji's project if Fuji also cloned Mew. This resulted in the creation of Mewtwo.
After the original Charmandertwo, Bulbasaurtwo, and Squirtletwo died during the experiment, Ambertwo—the cloned form of Amber—died as well, eliciting a heightened emotional response from Mewtwo. Dr. Fuji panicked and administered a form of emotion-suppressing serum to Mewtwo, which may have caused its seemingly-twisted personality. Time passed and Mewtwo matured, though shortly after gaining consciousness, it destroyed the lab on New Island, presumably killing everyone inside
(Source: Bulbapedia)
