We hesitate to put these two together, but, well, they are twins, so they're probably used to it by now. These two are more than just twins, though. They are "enhanced" products of Baron von Strucker's experiments. Pietro now has super-speed, while his sister has, well, supernatural abilities that manifest themselves as wispy clouds of red zaps. As it turns out, they're the only two to survive Strucker's diabolical procedures, but that really speaks to their motivation.
Vengeance Will Be Theirs
So what kept the pair going when everyone else died or quit? It was the prospect of getting revenge on Tony Stark and friends. Specifically, the Maximoffs' family was killed by a missile strike that used Tony's missiles. They figured this out when an unexploded shell landed in their living room. As they were trapped in the rubble, all the twins could see was Stark's name on the side of the device. As Wanda put it,
Wanda Maximoff: We wait for two days for Tony Stark to kill us.
Eesh. Yeah, we can see why they're not big Avengers fans.
Turnabout is Fair Play
All the same, the twins have a change of heart. When they learn that Ultron's out to destroy all of humanity, not just the Avengers portion, they realize that things have gone too far. Rather than helping the bad bot, they turn on him and join the Avengers.
This works out really well for them—right up until Ultron pumps Pietro full of Quinjet lead. The brother (known as Quicksilver in the comics) dies a hero, saving Hawkeye and an innocent little Sokovian boy. At the same time, he leaves his sister to exact the revenge they've been after all film long. She's not after Stark any more, though. She rips Ultron's mechanical heart out of his chest, striking a decisive blow for her family and loved ones. After the battle's over, Wanda pledges herself to become a member of the Avengers team. The Scarlet Witch (her comic moniker) is born.
No ruby slippers, though.