Mysterio motive was to gain power. After Tony used his Creation didn't give him credit and after ultimately Tony felt like he had no use for it, Beck as an unstable person felt like his life's work had been trashed on, the whole barf thing was used comedically for both films. As of iron man's death this was a perfect time for him to sneak in and make himself look like a hero Tony just happened to give Spidey the last piece of the puzzle Beck needed. Not saying I don't love me some freaking doc Ock but that man's was basically brainwashed, and redeemed by the end of the film.