While he retained a playful competitiveness with his brother, Loki looked ready to stand alongside him as a prominent figure wherever the remaining Asgardians ended up. Sadly, half of them ended up taken out by Thanos, shortly before the Mad Titan literally squeezed the life out of the trickster. As Thanos said, "no resurrections this time." So far that's proved true, albeit with the caveat of a different villainous incarnation of Loki being set to headline an Disney+ streaming series.
That said, while Loki may have been the designated villain of the first The Avengers, he arguably had plenty of motivation to become the bad guy in both scenarios. Putting aside the idea that Thanos was influencing Loki with the Mind Stone after a certain point, the younger Odinson had much reason to be bitter.
Was Loki Born A Villain (Or Did Thor Make Him One)?
While Thor matured into the hero he was destined to be after being sent to Earth as a test by his father Odin in the first Thor movie, and Loki tried to orchestrate Thor's murder, there are some important factors to keep in mind. For one, the whole reason Thor is banished to Earth is his egotistical, profoundly reckless behavior that risks the health and safety of both his entire family and every citizen of Asgard. Thor is extremely full of himself, and while not a terrible person, he's certainly very flawed. Given Thor's character flaws, it's no wonder that Loki would come to resent him enough to turn evil.
Odin had raised Loki since he was a baby. With magic used to conceal his true Frost Giant nature, and a spot as prince of Asgard, Loki should be pretty happy. Yet, he had to spend his entire life constantly in Thor's shadow, being upstaged by him at every turn. Watching someone that cocky be placed first in line for the throne regardless must have driven Loki nuts on a daily basis, and understandably so. Loki learns magic, cunning, and strategy, while Thor is content to just bulldoze his way through everything. Without Thor's negative influence in his life, it's entirely fair to argue that Loki never would've been pushed to the point of true villainy. Learning he was adopted may have pushed Loki over the edge, but dealing with a brother like Thor for untold decades beforehand walked him up to it.
