He watched his protégé and best friend, whom he loved like a brother, slowly turn bad. He fought his friend through lava flows that would burn away said friend's limbs and severely maim him in other ways, causing him to need a mask to breathe. I ask you, is this the action of a friend?
Then, Anakin out of the way for the moment, he contrived to separate Anakin's children and not tell his "dear friend" the joyful news that he was a father. He would spend the next few years as a hermit on a desolate island, only starting to be helpful when Anakin's daughter asked for help, and Anakin's son came to him with a droid with a message. Then he handed the whiny brat of a Luke a deadly weapon before telling him what it was. What if Luke had pressed the wrong button with the lightsaber pointed at himself? When that scheme didn't work, he made up some harebrained story about Darth Vader killing Luke's father - which he would later explain away by claiming it was true "from a certain point of view." Whatever.
Then, next time he saw old Anakin, he declared pompously, "If you strike me down, I shall become more powerful than you can possibly imagine." So, always willing to help a friend, Anakin took Obi-Wan's advice.
Like Yoda, Obi-Wan just disappeared. Like Yoda, I didn't see a body. Did he die? Like Yoda, he would show up later as a force ghost. He spoke in a creepy voice to Luke telling him to use the force. But were his actions as a ghost particularly powerful? Unless you consider spooky apparitions and voices powerful, not especially.
The last we see him as Obi-Wan, he is a holographic ghost smiling at the pawn he trained to defeat his old friend.
Fast forward about 30 years. A new generation has arisen, and now we see a powerful holographic ghost who is commanding his friend's grandson to commit horrible atrocities.