Nebula is one troubled character in the comic books and the MCU. The comic book version is exceptional in this regard. Thanos has turned her into a walking corpse after rigorous experiments and amputations. In an act of revenge, during the Infinity Gauntlet saga, Nebula catches Thanos off guard and steals the Gauntlet. She then undoes all the things Thanos has done, Thus the heroes gain a fighting chance to defeat Thanos for good.