Foul play.
My reasons:
1. Lights were off. Kinda strange, but not the most obvious.
2. If it was natural death, due to the way he fell, it can only mean he died in a circumstance that produce a death which didn't involve too much struggle it seems. Even so, a man who already has passed stool (stool was in the toilet and urine), should at least have left stains on the seat, assuming he did die from natural cause such as stroke or heart attack, or most likely bruises on his face/head or where he hit the ground.
Since it isn't the case where stool stains and bruises are seen, but since you mentioned---
" This particular morning..."
This can only imply that the person is female. One question then immediately springs up. Why did she decide to use the men's room, "this particular morning"?
The above fact along with the fact that she has a key, can only imply foul play.