Why did my headlights kill my car?

I think you have a short in the headlight cct. A bad h/light ground does not kill the entire electrical system.
I had a strange problem in my Dart years ago. HLs would get dim after wet weather. OK in dry weather. WTF?
The problem: before I got the car it had had a front end collision repair. The HL wires had been squashed between the rad support & bumper apron. The dust/pollution that settled there became conductive after getting damp & created a resistance short to ground; which slowly discharged the battery because the alt could not keep up with the current drain.