Dr. Stasis (a clone of Mister Sinister and a leader within Orchis) wanted to carve the magic of the Crimson Gem out of his physical body for the sake of further research and experimentation.

Source: https://screenrant.com/why-marvel-comics-killed-juggernaut-xmen/

For all the horror Mister Sinister has unleashed, he was only able to ruin the world by taking advantage of the failures of both mutants and humanity.

Source: https://www.cbr.com/humanity-abandoned-mutants-sins-of-sinister-marvel/

In order to end the conflict, rogue Eternals worked with the Avengers and Mister Sinister to create a new Celestial god for the Eternals, the Progenitor.

Source: https://www.cbr.com/xmen-greatest-victories-marvel-comics-2018-2023/

Mister Sinister made his play for power, creating a twisted alternate future where mutants took control of the universe at everyone else's expense.

Source: https://www.cbr.com/xmen-enemies-secret-weapons-fall-of-x/

Mister Sinister used the DNA of Cyclops and Jean Grey in the universe to create a powerful being named Nate Grey.

Source: https://www.cbr.com/marvel-iconic-characters-comic-event-debuts/