This is the problem when we attribute certain qualities to characters. Whilst we may not insert that they "always" did, that's how it's read. I have no doubt that Nevill was usually "careful about guns", especially so when the children were around. However, it appears, that on the night in question, there was 'stuff' going on. "Always" can't 'always' be guaranteed. From the general state of the kitchen, if Nevill was usually meticulous -even careful- he was the only person in the house who was, but it was "only" the kitchen, and, I suspect, a working kitchen with sundry comings and goings. It's not impossible that the rest of the house looked very different.