I grew up on an Army base in a foreign country before the Reagan administration where we dealt with very real threats on a frequent basis. Even as young children, we were taught due diligence. Be mindful of your surroundings. Be cautious, but not withdrawn. We were trained how to notice the littlest thing that was out of place and to calmly report. That way, when the bomb scares came in (and they did), the reaction was NOT terror, but calm and order.
I think the same applies to WDW or anywhere. Be aware of your surroundings and report any suspicious activity. Also know that no matter what, if someone enters your space with evil intent, there may be absolutely nothing you can do about it, whether it's WDW or your own local mall. You just have to continue to live your life as best you can, but with due diligence.