The Young Avengers didn't just come into existence to take the place of their predecessors when the original Avengers were disbanded. They were brought together for a very specific reason: to stop the rise of Kang the Conqueror. And the one who assembled them was... Kang the Conqueror. Sort of. In the future, a young man named Nathaniel Richards became aware that he would one day turn into Kang and was horrified by the evil he foresaw. Unwilling to accept it, he traveled to the past to enlist the Avengers' aid, but only found the remains of their work. But that led to the formation of the Young Avengers and, eventually, his descent into villainy.
Nathaniel Richards was pulled out of time by Kang to prevent him from receiving an injury that would have hospitalized him for years. Kang showed Nathaniel all that would come to pass when he became the conqueror and implored Nathaniel to kill those who would have been responsible for injuring him, potentially setting him on the path to becoming Kang much earlier. But this plan backfired. Horrified by his future and the atrocities he would commit, Nathaniel instead stole time travel technology from Kang and fled to the past.
He tried to find the Avengers to help save him from his fate, but discovered that the time period he had fled to was when the Avengers were disbanded. Seeking answers, he downloaded the remains of Vision into his armor and discovered the protocol to locate the next generation of Avengers in the event the team was not available to save the world. Using that information, he formed the Young Avengers under the alias Iron Lad to prevent his turn to villainy and potentially do some other good along the way.
His time on the Young Avengers was short-lived, however. Iron Lad established a romantic relationship with Cassie Lang, but before that could go anywhere his older self appeared and instigated a battle between himself and the Young Avengers. Knowing that Kang would drag him back to the future so he could fulfill his dark destiny, Iron Lad took drastic action to stop him. He used Hawkeye's sword to kill Kang, but this created more problems than it solved.
The ramifications of Kang's death affected the entire timeline. In the new history, the Avengers have all died, Wiccan and Hulking of the Young Avengers no longer exist and Jessica Jones had lost her baby. Seeing the damage that his fear of the future had brought, Iron Lad realized the only way to restore the timeline was to go back to the future and become Kang as history intended. He left his teammates, asking them to forgive him for whatever he does in the future.
But that wasn't the choice that finally turned him into a villain. Iron Lad returned during Avengers: The Children's Crusade by Allan Heinberg and Jim Cheung, just in time to watch Cassie die at the hands of Doctor Doom. Grief-stricken, he resolved to make corrections to history to save her and killed the young Vision when he protested against the abuse of time travel technology. His teammates couldn't stop him, but they tried to make him see that this course of action would most likely turn him into Kang. He refused to listen and departed for times unknown.
The Young Avengers were created to stop the rise of Kang the Conqueror. But in the end, they became the motivating factor for Iron Lad's descent into villainy. He cared for the people he brought together to stop himself, but that love overwhelmed any fear he may have had of the crimes he would commit in the future. So in the end, Iron Lad became Kang for love.