Saw someone with their gun exposed and I let them know. They were irate

So New Years Eve my girlfriend and I stopped at Costco for a few last minute items. As we're walking out I'm beside this older guy, maybe 50s, and his wife. He's wearing what I can only describe as as sort of like a baseball coaches outfit, but casual. A pair of those vinyl sweatpants and a matching windbreaker likethis. Whatever, but he has a Shield with a utiliclip at his 1 o'clock with the windbreaker tucked behind it, so it's totally exposedFirst off, we're in a really gun unfriendly state. Second, Costco has a store policy against guns so I don't want this guy getting banned or in trouble, and third, I've been told I'm polite to a fault. I like helping people out if I can. Plus seeing a fellow carrier around here is refreshingI walk up to him on his right and say "Excuse me sir" while putting my hand up as a sort of wave to catch his attentionHe jumps a little and does that thing where you put your arms to the side like you're about to throw down like this Spongebob meme and keeps his right hand against my left bicep/shoulder, I guess to keep me at a distanceHim: What, what, what do you want?Me: Sorry, I just wanted to let you know you're printingHim: What?Me (Lowering my voice): Your gun is showingHim: That's none of your fucking businessMe: Sir, I just wanted to let you knowHim: I can do that. You don't know me, you don't know what I doAt this point I assume he's a cop or a CO. And technically, despite being unfriendly to guns, it's an open carry state so anyone can do thatMe: I'm sorry, I just wanted to give you a heads upHim: It's none of your fucking business. Why'd you sneak up on me? I can do thisI look at his wife for a second to see her reaction but she's unfazed. I have no desire to be here anymore because he's actually shouting and making a sceneMe: Alright. I was just trying to be helpfulHim: Don't fucking sneak up on me again! Don't do it!I'm always worried about printing because of how gun unfriendly this state is, so I'd want someone to let me know if I was showing. And it really bummed me out because I thought I was doing something nice, I didn't want him getting the police called on him or someone less friendly noticing. Did I do something wrong here? His reaction was actually baffling. As far as I'm concerned, it would be no different than telling him his shoe is untied or his shirt was tucked into his underwear via /r/CCW https://ift.tt/395syS7