There's a lot of, frankly, bad speculation about Disney's Security operation going on in this thread, and as someone who works in the Security Industry, I wanted to throw my two cents in.
-Disney Parks are what are known as hardened targets. There are multiple layers of security between a potential perpetrator and potential victims. There is no such thing as immunity from attack, but Disney very clearly takes all the necessary steps to protect park guests.
-A lot of people have complained about the "security theater" at the parks. Security theater, while not always the best option for several reasons, offers a valuable deterrent effect by forcing every guest to have direct interaction with a member of the security team. Same goes for bag checks. Yes, there is value in searching people's property for illegal items and contraband, it also offers an opportunity for a face to face personal assessment by a security professional trained to observe people for unnatural or suspicious behavior. Interactions with Security Cast Members offer the opportunity to stop events from happening when they're in the planning stages, as opposed to when the shooting begins. It's much easier to deter an active shooter from choosing you as a target than it is to stop them as the incident is happening.
-Disney has a technological advantage over most police departments in the US. Not to mention the fact that they're not bound by pesky things like the 4th amendment. There are layers of security inside and around the parks that the average person will never know or care about. And WDW especially has a fantastic tool for tracking anyone inside their parks. Think your MyMagic+ is just there to offer you a seamless payment and ticketing experience? Guess again.
-Just because you can't see the metal detector doesn't mean there isn't one. Just because you didn't walk through one doesn't mean you weren't scanned. On my recent trip to DLR (September), everyone was going through traditional scanners, for what it's worth.
-It's easier to spot a security risk from inside your own walls than it is from the outside. If someone's actual plan was "go work for Dis, gain the trust of my coworkers for 3-5 years and then go nuts" they'd be found out in pretty short order. No legitimate psychopath is going to be able to mask their intentions for that long. Now, if you want to argue that disgruntled workers in general are a risk, I'm happy to agree with that. That's why Disney (and frankly every employer) needs to be more committed to providing safe and nurturing work environments that minimize psychological distress and offer opportunities to grant treatment to at risk individuals. America in general does a really bad job of this, but improving working conditions even slightly can have tremendous benefits in risk reduction.
I'm not stupid enough to pretend the risk is zero, or somehow negated entirely by the security operation on Disney property. But they do a much better job than a lot of the people in this thread are giving credit for. There's always risk, Vegas proved that beyond a doubt. But Disney does an excellent job. They're the industry standard, bar none. And if you're still concerned, I honestly have to recommend that you stay home, because if that's the case there's nothing Disney could ever do to make you feel safe.