|Showing up at work and basically sporting an advertisement for the competition is simply asking for trouble. Here is an easy way to look at this: many companies envision themselves as teams, and you showed up wearing an opponent's jersey. What would happen to a member of the New York Yankees who showed up for a game wearing a Boston Red Sox jersey?|
You mentioned this was not a joke, but perhaps you could have gotten away with this if it were some sort of joke. By thinking that your behavior was not a big deal, you have inadvertently sent a message that points to a lack of understanding of the business and industry, disloyalty to the company, lack of pride in the company, insensitivity to your fellow employees, and real questions about your judgment.
You also mentioned that management's reaction is a sign of insecurity, but all that has happened is that you have undermined your own job security. Employers prefer employees who would give them the shirt off their back, and that's exactly what you should do in all respects.