Nemesis is the physical embodiment of all six Infinity Stones. She was chilling, just doing her then, but then, get this, she got so tired of living that she just burst herself into six stones and scattered them across the galaxy. That is the ultimate case of "I'm over it" that I've ever heard of. Imagine being at a party that sucks, so you suddenly just burst into gems. I think everyone would take the hint at that point. Your party sucks, bro.
You can't blame some of these eternal beings for getting bored with life. When you're super strong and can live forever, what are you supposed to live for? No wonder so many of them are either hell-bent on killing things, or constantly fighting each other using elaborate plans that span decades and galaxies.