Thanos has saved the universe just as much as he's tried to kill a bunch of people and he's never actually tried to destroy it... all he wants is some love from Death. He also helped seal off the cancerverse, was a member of Infinity Watch, and had so much respect for Mar-vell/Captain Marvel he helped him ease as he passed.
Besides his genocidal tendencies, deep down, he's actually just a misunderstood child.
Basically the cancerverse and the 616 universe come into contact and the 616 universe starts being flooded with too many beings... the cancerverse is acting like a universe that grows like a tumor that won't freaking die. Thanos, Gamora, Star-Lord and... Silver Surfer I'm pretty sure... They got trapped there. Thanos actually had Mar-Vell kill him so he could die in the cancerverse and unleash Death.
Besides his genocidal tendencies, deep down, he's actually just a misunderstood child.