In 2012, Marvel Studios concluded Phase 1 with The Avengers, bringing together Earth’s Mightiest Heroes against the ultimate villain: Loki. But what many fans didn’t realize at the time was that the trickster god wasn’t entirely himself throughout the film. As it turns out, he was under someone else’s influence.
The Scepter, gifted to Loki by the Other via an energy portal unwillingly emitted by the Tesseract, was actually a mind control device. This is why it was able to amplify the emotions of those who touched it, which is why it worked on Thor and others. It was also able to affect the minds of those who interacted with it, which is why Loki was able to use the Scepter to brainwash Clint Barton and Erik Selvig.
But this doesn’t mean that the Scepter made Loki a villainous maniac, just that it fueled the rage, hatred, and greed that already lived inside of him. It was just a tool to take his evilness to the next level, and it worked.
I know that a lot of people are getting all bent out of shape about this, but it’s not like the Scepter literally mind controlled Loki or something. It just influenced him to do bad things, which is really what everyone expected from him. And that’s why he was so much worse in Avengers than he had been in the three Thor movies, because his petty, vengeful, and dangerous tendencies were magnified.