Agreed, but the mass murders that we've seen in the past several years do not typically (I'm not aware of any cases at least) involve lock circumvention.
Additionally the process of circumventing a lock requires time, effort, at least a little practice and familiarity with the tools, which most of these guys do not invest into their murder sprees. Whether a lock can be broken is a different matter from whether a lock will slow down an assailant unless the lock offers no resistance.
I agree that there hasn't been a precedent for mass murderers circumventing locks, but that's partly due to the fact that mass murders typically occur in public, where doors are kept unlocked for the most part.
Some lock circumvention techniques take almost no skill, and no time at all. I've seen cipher locks that cost thousands circumvented by lightly tapping the housing with a rubber mallet or sticking a paperclip through a gap near the LED to create a short. I've seen people open locks with a piece of toilet paper or a key that was milled using a photograph taken of a key-chain on someone's belt from across a parking-lot.
Shooter has his gun(s) out, he approaches door, he now has to holster gun, get out bump key, focus on and fiddle with door, and once it's unlocked, open up the door, unholster his weapon, and then enter the door.
How many times do you think the shooter is going to expose himself like that? Does that feed into the fantasy of stalking people through the hallways shooting them?
Soldiers used to struggle with this issue, but someone (probably a Soldier) invented a new type of sling that eliminates it. With the old slings, basically every time you needed to use your hands, you had to sling your weapon on your back, but the newer version keeps your weapon suspended in front of you, a little lower than chest level. All you have to do is let go, and your weapon stays exactly where you need it.
If a shooter is using a handgun, well with just a little bit of practice, you can learn to draw a handgun in a fraction of a second.
Basically, I don't think it's a good idea to assume what will or will not be reasonable in the eyes of a mass-murdering lunatic.
Additionally the process of circumventing a lock requires time, effort, at least a little practice and familiarity with the tools, which most of these guys do not invest into their murder sprees. Whether a lock can be broken is a different matter from whether a lock will slow down an assailant unless the lock offers no resistance.