A guilty Wallace would have had to make absolutely sure he had killed Julia, so the number and force of the blows could have been a combination of repressed rage, finally coming out after all the careful planning, plus the need to ensure they would be fatal. She must have been a tough old bird in some ways if she could pass for a much younger woman.
Leaving very little blood or mess outside the parlour; turning off the downstairs lights; replacing the cash box; and finally removing the murder weapon from the house, all point to someone in the process of carrying out a premeditated crime, but one whose attention to detail was a character trait not easily deviated from, even when his 'tidiness' would have seemed inappropriate for an intruder.
Had there been a trail of blood from parlour to back door, for instance; had the lights been on and the cash box left open on the floor; and had the weapon been left, but with no fingerprints, this would all have been consistent with an intruder wearing gloves.
I don't believe this crime was committed by an intruder who went out of his way to make it look
like Wallace, being too fastidious for his own good. It's simpler and makes more sense to see this as Wallace himself, being too Wallace-like for his own good.