Time travel in superhero stories often comes with a number of complicating factors. Changing the past can have plenty of unintended consequences, with characters from the DC Universe (most notably the Flash) finding that out the hard way. In the Marvel Universe, the X-Men have encountered plenty of time travel, and are well aware of the effect changing the past can have on the timeline. But when given the chance, one X-Man veteran was more than happy to change the future for the better by eliminating one of history's worst figures.
X-Men: True Friends (by Chris Claremont, Rick Leonardi, Al Williamson, Jimmy Palmiotti, Shannon Blanchard, and Tom Orzechowski) saw Kate Pryde and Rachel Summers get transported to the past, in the days before World War II. While other time-displaced heroes would be more careful with their time in the past, Kate Pryde wasn't interested in protecting the timeline. Instead, the X-Man planned to go on a killing spree to wipe out Adolf Hitler and the rest of Nazi leadership -- and would have created a wildly different Marvel Universe as a result.
How Kitty Pryde Was Sent Back in Time
X-Men: True Friends focuses on Kate Pryde and Rachel Summers as they take a trip through the Scottish Highlands. Rachel's vast psionic abilities and connection to the Phoenix Force interacted with the lingering magics of the era and sent them spiraling back in time to the 1930s. There, the pair discover a plot by the Nazis and their newfound ally the Shadow King to destabilize the British government ahead of an invasion. While Rachel was captured and briefly brainwashed by the psychic villain, Pryde forged a bond with the heroic Alasdhair Kinross and eventually worked alongside the Logan of this time period to prevent the Nazi scheme from going into effect. However, Kate decided her time travel had given her another opportunity she couldn't afford to miss.
Using her knowledge of the future and her mutant abilities, Pryde decided she could hunt down the Nazi leadership and eliminate it. By doing so, Pryde could eradicate the architects of the Holocaust years before the genocide went into effect. Revealing the source of her knowledge to Alasdhair, Pryde proved unflexible in her intentions. When Alasdhair argued that changing the past might radically alter the future, Pryde's morality prevented her from simply turning her back on the opportunity to save millions of lives. Ultimately though, her plans were side-tracked when Alasdhair convinced her to help defeat the Shadow King before embarking on her mission which ended up shunting her and Rachel back to their proper place in the timeline.
Kitty Pryde Nearly Assassinated Hitler
It's notable that while Kate Pryde had the idea to hunt down the Nazi leadership, every attempt to dissuade her was promptly ignored. The moral quandaries of her breaching an ethical line pales in comparison to the lives she could save, and the impact on the timeline isn't enough of a risk to dissuade her. Even the Logan of this period is taken aback by her willingness and desire to hunt down and eliminate the Nazi leadership but notably doesn't try to convince her beyond that. It's pure luck that kept Pryde from radically altering the future of the Marvel Universe, and trying to reshape it in surprising ways. This makes perfect sense, given Pryde's willingness to be ruthless to take down genocidal monsters.
On the one hand, it likely wouldn't have had an effect on the core Marvel Universe. Attempts to alter the future in the Marvel Universe often result in divergent realities instead of changing the full course of history, as seen with another time-displaced Kate Pryde in Days of Future Past future. But Pryde's actions might have resulted in a wild cascading of events. Eliminating the Nazis early would have prevented their crimes, but conflicts around the globe and the rise of authoritarians in Italy and Japan wouldn't have been simply halted by Pryde's successful assassination of Hitler. Kate's actions might have prevented World War II, which in turn would have prevented Steve Rogers from ever having a reason to become Captain America -- negating the role he would have in saving the world over the years. Pryde's actions might have created a timeline where unforgivable crimes were prevented, and where the Marvel Universe looks radically different.