Anakin and Padme's marriage was one of the best kept secrets of the Star Wars prequels, with them going through the entire Clone Wars with no one ever finding out. However, what if Padme was keeping a far worse secret? As some hints suggest that she might've been having an affair, and with none other than Anakin's master himself -- Obi-Wan Kenobi.
On the surface, this theory sounds preposterous, as why would Padme ever do such as thing? No matter the opinions on her and Anakin's love scenes during the prequels, it's clear that the two have a deep love for each other. Yet the theory isn't as far-fetched as it may seem, as even Darth Vader has had subtle suspicions about Obi-Wan and Padme's relationship.
During Star Wars: The Clone Wars, it's revealed that Obi-Wan is no stranger to love, as he too once had someone he deeply cared about. But knowing that love is forbidden within the Jedi Order, he chose to part ways and remain faithful to the rules. But that doesn't mean he wasn't allowed any intimacy at all, as the Jedi simply aren't allowed attachment.
Because of Anakin, Obi-Wan and Padme were somewhat close. He knew her well enough to piece together their relationship, and went straight to Padme after Anakin had turned to the dark side. Together, they left to confront Anakin on Mustafar, and Obi-Wan stayed with Padme all the way through to her death. And so, it's quite possible the two were more acquainted than once thought.
But even if the two did or didn't have an affair, Anakin may have still suspected it. All throughout the Clone Wars, he was known to be incredibly possessive and jealous. Anakin even came close to killing a politician named Clovis, as he held a deep love for Padme, and would frequently make moves on her. He not only attacked to protect Padme, but also because he was worried she would fall for the politician.
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Then much later on, when Anakin had fallen to the dark side, he was surprised to see Padme meet him on Mustafar. Anakin was somewhat happy to see her, that is until Obi-Wan walked out of her ship. He yelled, "You're with him! You brought him here to kill me!" -- which suggests Anakin believed the two had been plotting against him. And if he did believe Obi-Wan and Padme were having an affair, it would explain why he so quickly chose to strangle his wife and attack his master.
And this isn't the last time Anakin suspected something, as during the Star Wars: Darth Vader #5 comic, Vader is seen visiting the medical facility where Padme died. Here, an accompanying droid points out that a ship with both Obi-Wan and Padme once arrived here, with Vader simply replying "Kenobi..."
The droid then shows a hologram of Padme laying on her deathbed, with one of her final words being "Obi-Wan..." This only makes things worse, as the droid adds, "That name again! He must've been very important to her!" But silently, Vader contemplates and simply leaves, suggesting he had thoughts about how close his old wife and master actually were.
While it's still hard to believe that Obi-Wan would do such a thing, it's not out of the realm of possibility. The theory could even explain why he watched over Luke for so many years, as he would've felt guilt for his past mistakes. But either way, with Anakin's constant jealousy, he very well might've had suspicions all along. Which only adds more weight to his tragic turn, and makes his "I hate you!" comment to Obi-Wan cut even deeper than before.