The reason that middle-age Biff is no longer mean is because he's basically been beaten into submission by George. Although it's presented to us as a "happy ending", with Biff getting what he deserved etc., from the perspective of Biff and George's characters, their roles have simply been reversed. But Biff's underlying villainy is always there, and it comes out at every opportunity.
In Back to the Future I, Biff is still the same personality he always was. He's just now scared of George, and had his reputation ruined by it, and it obviously ruined his life as a result. He's stuck working for the guy that beat him up at prom, with no way out. George, for his part, doesn't seem to have many qualms about treating Biff poorly.
In the beginning of Back to the Future II, future-Biff has now spent a lifetime being abused by George McFly. In 1985, he saw Marty and Doc fly off in the DeLorean, so when he sees them again in 2015, he knew something was up. Once he figured out about the time travel, and heard Marty's plan to use the almanac, he saw a way to get revenge on his tormentor. There's also a decent chance that future-Biff had started putting all the pieces together: he's now seen the same guy -- Marty McFly -- in three different years looking exactly the same. However cranky he was before, figuring out that Marty was responsible for all his problems is only going to escalate his evilness.
Future-biff then goes into the past to provide past-Biff with the means to escape from that destiny. By allowing past-Biff to get rich, he no longer needs to grovel to George. Biff once more has power over "lesser" people; only now, he's been embarrassed by someone he used to consider insignificant. So he's gone from a moderately-dangerous high school bully, to a filthy rich, vindictive high school bully.