Thanos #14 got me thinking when it showed Thanos as a baby in his crib on Titan. If Norrin and Nova were evil and considered all options, would Thanos stand a chance? Consider that Surfer and Nova combined speeds in Surfer #51 to travel tens of thousands of years back in time. Does Thanos have a failsafe plan to protect him from time travelers murdering him as a baby? What else might Norrin and Nova be capable of that might defeat Thanos, if anything else?
In the MU going back in time simply causes a bifurcation and creates another reality. So in that reality Thanos as a baby is dead...in their own it has no effect. Only one time travel trip was otherwise...a time machine created by Tony Stark to specifically not cause a bifurcation.