Killing his parents? That's a pretty important detail that this girl left out. Also, does it make you uncomfortable that they know so much about- I don't think it's right, to be so open about other people's business.
But fine. Following that thought- Pietro and Wanda join Hydra because they want revenge against Tony Stark for killing their parents. Fair enough. Tony Stark creates or finds or gives rise to Ultron, who I assume is intent on destroying the world because that's what killer robots do. The Maximoffs team up for revenge, but- the incident is in Sokovia, right? Their home. They realize they don't want it destroyed, and switch sides. Pietro dies. Wanda joins the Avengers because she has nothing left. Except there's still the matter of the dead parents, a dead brother, and who is to blame for Ultron. Looks like Stark is insisting Wanda made him do it?
Now Pietro is back from the dead. But why Clint, Banner, and Rogers? Why not Tony himse- oh. Oh. Because Tony took his family from him, right? He's going to make Stark watch as he does the same.
It'd make for a great mystery novel, at least. The motive checks out.
If Pietro still had his powers, it'd make sense. If there's multiple murders going on, all over the place, at about the same time, I'd look at a teleporter or somebody very fast.