How Star-Lord Killed Thanos (And Why It Didn't Matter)
Star-Lord has successfully killed Thanos in the comics, however, it wasn't a big victory like one might think. After using the Cosmic Cube to incinerate Thanos while stuck in the Cancerverse, the Mad Titan immediately returned and made sure to dish out his own killer blow on the leader of the Guardians of the Galaxy.
Peter Quill famously botched the Avengers' attempt to take down the Infinity Gauntlet-wielding Thanos in Avengers: Infinity War, as his emotional reaction about Gamora's tragic death got the better of him and his anger allowed the Marvel villain to break free and eventually snap half of all life from existence. But, in the comics, Star-Lord actually had the chance to kill Thanos and didn't screw it up - but the death was short-lived as the Cancerverse meant death wasn't as permanent as it would be outside of it.
In Guardians of the Galaxy #19 by Brian Michael Bendis, Ed McGuinness, Mark Farmer, Mark Morales, John Livesay, Jason Keith, and VC's Cory Petit, Star-Lord, Drax (who previously died) and Nova successfully trapped Thanos in the Cancerverse - however, everyone but Nova, including Thanos, managed to escape. When recounting what happened inside the parasitic dimension, Peter recalls using the Cosmic Cube to successfully incinerate Thanos. The comic shows the Mad Titans' bones disintegrating as the powerful blast hits him. Star-Lord, Nova, and Drax celebrate finally taking down Thanos for good.
However, it's soon revealed that nobody can die in the Cancerverse, as Thanos returns and punches his fist right through Peter's chest. After Nova and Drax kill Thanos again - and Peter returns from his own death - the villain reveals since they weren't all born in the Cancerverse and aren't made up of materials from the dimension, "we cannot die here." So, instead of Star-Lord and company beating Thanos, they're effectively stuck in dimension where they can die, but be reborn, over and over again. Nova would be trapped for quite a long time.
Unlike his MCU counterpart who ultimately flubbed The Avengers efforts to takedown of Thanos, Peter Quill was able to defeat Thanos in the comics. However, his death was only temporary meaning that his demise didn't matter in the grand scheme of things as he was quickly brought back to life. So, while Star-Lord had to feel good using the Cosmic Cube to kill Thanos, it was only shortlived as the Mad Titan would eventually escape the Cancerverse (and from getting killed again) and terrorize the Marvel Universe once again.