He relapsed. He does bring this up when he's talking to Cap in Civil War, in the end he just wasn't able to step away from it all.

He's seen what's coming; he saw the alien ship in Avengers 1, and then he saw what he interpreted to be visions of the future when Wanda messed with his head in Avengers 2. As Thanos comments, he's been "cursed with knowledge". And Tony believes himself to be the only person who has both the willpower and the means to actually do something about it.

It started with the Iron Legion, which were just a series of non-combat-based peacekeeing drones. Creating Ultron probably wouldn't have been a big leap in his mind, because after all Tony has successfully created AI before in Jarvis. And now, having seen the kinds of threats that are imminent, he's naturally returned to doing what he knows; being Iron Man.