It Just Gets Harder and Harder
In our view, Marvel did an outstanding job crafting the perfect supervillain in Loki. Thor's mischievous brother was witty, formidable, and downright evil. He happened to be an actual god, just like Thor. However, fast forward a few films, and we're introduced to Ultron, who clarifies that he's the biggest, worst entity around.
As if Ultron weren't enough of a challenge for the Avengers, Marvel then introduces Thanos, who can and did turn half the universe into dust with a mere snap of his fingers. What's next, an indestructible killing machine that claims the lives of the remaining superheroes?