Robin Westman -- the Minnesota catholic school mass shooter -- asked security guards at the dispensary where he worked about guns prior to his murderous attack -- and was also obsessed with playing super violent video games ... TMZ has learned.

A former coworker of Robin's at a Rise medical cannabis dispensary tells TMZ ... Robin started with the company in late March, and the two worked together up until a week before the shooting -- but Robin never really let on about the deadly plan, yet there were signs. TMZ.com

Our source says although they never heard Robin bring up guns at work ... one coworker told them in the aftermath of the shooting that Westman asked the security team about firearms, though it was not clear in what context.

We're told Robin -- who used they/them pronoun

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