Needlessly complex explanation.
Obi wan's purpose was to guide and protect Luke.
He could better fulfil this purpose by being killed by Luke's dad (uhh, spoiler alert, sorry) right in front of him.
Fulfilling his purpose makes him stronger.
Therefore being killed made him more powerful than you can possibly imagine...
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soooo destiny = star wars?
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I mean, technically, it is a war over our solar system. Solar = a star. Star Wars.