Elon Musk says fans swarming him for a selfie aren't the only reason it's nearly impossible for him to go out in public ... he told "The Katie Miller Podcast" Charlie Kirk 's murder raised the stakes to life and death.

Watch the video ... Elon tells Katie, "These days -- particularly in light of Charlie Kirk's murder, there are serious security issues." Getty

According to the world's richest man, his friend's assassination reinforced the severity of the situation, making him more reclusive. He couldn't seem to recall the last time he'd been out on a routine errand like a grocery store run or a stop at a drugstore.

"It's not that I don't want to," he said. "I simply can't."

Elon explained he always has to have his guard up, for fear of the worst ... "Life is on 'hardcore

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