WARNING: The following contains spoilers for What If...? Season 1, Episode 3, "What If... the World Lost Its Mightiest Heroes?," now streaming on Disney+
In the Marvel Cinematic Universe, the Yellowjacket Suit is a heavily weaponized variant of the Ant-Man suit, introduced in Ant-Man. A creation of Darren Cross', the suit would theoretically create one of the deadliest assassins in the MCU. What If...? just revealed how dangerous someone like Hank Pym could be in that identity -- with one entire reality up-ended by his decision to steal the suit and use it to target Nick Fury during the week where he took a series of major steps towards recruiting the original MCU Avengers, murdering the candidates in cold blood.
But this isn't the first time that Hank Pym becoming Yellowjacket was a portent for dark times -- with Hank's original version of the costumed identity being one of the first big hints towards the instability he would go on to suffer from in many of his future appearances in the Marvel Universe.
In the core-Marvel Universe, Hank Pym was one of the founding Avengers as Ant-Man. Alongside his love interest Janet Van Dyne -- aka the Wasp -- he became a major figure in the franchise, briefly leaving the team only to return in Avengers #28 by Stan Lee and Don Heck as Giant-Man. But a series of setbacks and mishaps began to take a mental toll on Pym -- most notably his creation of Ultron, which quickly evolved into one of the most fearsome threats that the Avengers would ever encounter. His endless habit of working non-stop eventually resulted in an accident, as revealed in Avengers #59 & #60 by Roy Thomas and John Buscema.
In that story, Hank Pym proved incapable of diverting his attention fully to Janet, increasingly frustrating her. But while alone in his lab, Pym was accidentally exposed to unique and untested chemicals -- affecting his mental state even further and causing a personality crisis. Fixated on Wasp but furious that Giant-Man wouldn't marry her, he decided to create a new persona who could -- leaving to the invention of the Yellowjacket identity. In Avengers #59, Pym confronts the Avengers in this guise and demands acceptance into their roster -- even bragging that he'd actually killed Hank Pym in an earlier encounter by trapping him in his shrunken form and leaving him to be devoured by a spider.
Wasp -- realizing the truth about Yellowjacket's identity but keeping it a secret -- goes through with his plan for a wedding, leading to a brawl between the assorted heroes and the attacking Circus of Crime. The wedding was held up as official, and the now-married pair continued to be heroes for a time. But Pym's instability in this storyline was be something of a precursor of his eventual evolution. Hank would fluctuate between his Ant-Man, Goliath, and Yellowjacket identities over the ensuing years -- and was in the Yellowjacket identity in perhaps his most infamous moment, striking his wife in the midst of a mental breakdown. This in turn led him to plan a fake attack to endear him further to the Avengers and was expelled from the Avengers (and divorced by Janet) for his actions.
In the present-day, Pym is missing following his fusion with Ultron -- with the indication being that the act of "fusing together" had more or less killed Pym. Many of the pains Pym has gone through (and forced his loved ones to endure) stem from that period of Hank's life when he became Yellowjacket. Almost all of those dark elements of Hank Pym can be traced back to his time with the Avengers and the instability that followed, showcasing just how far gone Pym has ever been to fully embrace being Yellowjacket, seemingly regardless of the universe it takes place in.