Should a bathroom sink drain outlet flow away from the trap, or back toward it?
It definitely needs to slope away down to the drain. Specifically a quarter inch per foot. No more, no less. The reason for this is that at a quarter inch per foot waste water tends to sort of oscillate up one side and then the other of the pipe. This effectively washes the pipe as it goes. If you have a negative slope any debris will have plenty of time to firmly attached itself to the bottom and side walls. Too much slope in the water shoots down center of the pipe and debris build-up into lumps on either side of the pipe.