The Infinity Stones are a set of six elemental powers in the Marvel Universe. The Stones were created during the Big Bang and possess unparalleled potential. Even a single Infinity Stone is a terrifying artifact. For example, the Time Stone allows its wielder to manipulate the passage of time.

After losing everything, Vader blamed the Jedi for his wife's death and his disfigurement. Without Padme or the Order he was a lost and broken man. Vader’s allegiance to the Empire was necessary; he wasn’t physically or mentally strong enough to defeat his new master.

Had Vader acquired the Infinity Stones, the galaxy would've been in trouble. With the power of the Stones at his disposal, he could have easily overthrown Palpatine and installed himself as the leader of the Empire.

Vader didn’t fall to the Dark Side out of a desire to rule. He started down his cursed path to protect Padme from an early death. In fact, everything he did in Episode III was to keep her alive. He’d bring her back in a heartbeat if he had the chance.

However, using the Stones in this way would likely result in Vader’s death. Thanos and the Hulk only barely survived using the Infinity Gauntlet, and both have a unique reason why. As strong as Vader is, he’s not superhuman. His use of the Stones to reverse Padme’s death would likely have the same result as Tony Stark’s final act as Iron Man.