Kettle fillers are bizarre but I think there's slightly more to it. Some people think they are being rude if they only prepare enough food for themselves. I think kettle fillers may believe they are doing a service for others when they fill. But that's really stretching the benefit of the doubt here. They may just be thick.

I can actually top this, though. In my student days I had a guy from a hot country move in. Apparently the water from the tap was too cold so he would warm it in the kettle. The trouble is the kettle is too hot... So I watched in utter disbelief as he boiled the kettle, poured boiling water into a pint glass, then carried said glass to the fridge to cool it back down to drinking temperature!

Later I came back and he'd forgotten about the water so it was now colder than tap water in the fridge.