A great question. If I have this correct, there was testimony they were on very early but not so later? And in any event the witness had found that odd?
So, it could have been the case MY had left the lights on, if she had been worried, but I think that is contrary to what is believed - which is the lights were turned on at some point during the night/early morning.
It was a dangerous move regardless of the killer's identity. To support my opinion, I would say JY knew no one was coming home, knew he had a bigger mess to deal with than he had planned, knew he had to minimize the evidence, and so turned the lights on to deal with it since he was in a hurry. To the contrary, the same could be said for the stranger killer, and to boot that the stranger would more likely need the lights on not being as familiar with the home.
The lights were on and never turned off.