You have the human killer at your finger tips mate. A horse goes beserk, and the grooms advice you of this fact. The grooms goes on oxygen, you dump the cabin pressure and the nags to go sleep. Problem solved.
Using a .45, or a deadbolt, will render several million Euro's worth of horse rather dead. That will not do; them horses are far more expensive to replace than you and I mate
Will there be a humane killer onboard? I suppose so. It will be under the command of either a vet or the chief groom. Will it be used? Not unless you can't dump the cabin fast enough!
As for the reinforced door ... Well, it doesn't actually improve communication between cockpit and cabin does it? On the horse fligths I've loadmastered I seem to have been constantly going from cabin to cockpit conveying information (can we descent to escape turbulence? - when are we landing? - can you fly around them towering CBs? - are the horses behaving? - have you briefed the grooms/vets about the landing - can you spare me a fag? etc etc etc). Not exactly crucial information, but something that is very much appreciated by the grooms. And they are our costumers, and if they should develop negative feelings about flying with DHL, they've got the power to make the contract go to someone else. The power they hold is the sole reason that a few of the rear windows will be uncovered both sides; to let in a bit of sunshine which will make them happy.
The good thing about a door is that it can be left both open and closed, if you catch my drift ....