What do you suppose she went back for? *spoilers*

Actually this is much more subtle than you'd think.

Her last words are "Jill, Where are You?". Jill says the key is there, that that phrase has meaning. Julia needs Jill. Not just at the present moment of the shooting but because Jill was needed before. At her apartment.

Up until that point in the film Jill and Julia are operating under the assumption that William Landis has no idea where Jill lives or that she's involved and that she's "safe" there. We've all seen the end of the movie so we know this is false. Jill's last words are indicative of panic, she needs somebody. Why would she need somebody and not a place to stay?

Because Landis probably made an appearance. Landis only blocks the front door for Jill who he presumably is stalking to tie up a lose end. When she realized his presence was there she left immediately. The police don't believe her and she's all alone. She returns to her place for at least three reasons.

1. The gun is there.
2. She has nowhere to go that is safe.
3. When Jill comes home and finds no one there she'll come to Julia's place automatically.

The message on the note says "It's dark". The bullet wound was at her temple presumably where he shot her to make it appear as a suicide. She says it's dark because she knows he's there but she can't see him. He's camouflaged probably near her bed. She can't call Jill for help which she needs, direly, because Jill's on the road heading towards the house from the beginning. Jill on the phone call earlier gave her a list of directions for securing a house. She's repeating those because she needs a secure place to be and why would she need that? Because Jill's place is no longer secure. Landis HAD to have made an appearance if she was struggling to secure her much smaller, much more controllable apartment.

Jill also wrote in that book every time Landis came by. She may not even have known he was there at the apartment other than the fact that the lights had been disconnected. The important part is that she was trying to secure a location - which meant Jill's house wasn't safe.

Call me. Turn on the lights. Look around. Bolt the window. Clear the kitchen, clear the bathroom, clear the closet and then bolt the front door. Julia wants to repeat these steps but the first one she can't. "It's dark" means the lights have been broken.

She's shot in the middle of the room away from her bed so presumably she went to the kitchen table to retrieve the gun. Obviously Landis was waiting for this move as he waits Jill out similarly at the end. He grabs her and uses her own weapon her similar to how Jill' weapon was taken from her after a struggle - it probably went easier with Julia because Julia had no physical training.

Lastly there's one more suggestion. William Landis could've written it himself after overhearing her conversation about securing a living space.