if jeremy hired a hitman then theres no evedence to say the hitman did stick around to wait for the police.
if we go on sherlocks secound thory that it was somone trying to lure jeremy ovrr there then they wouldent know jeremy was going to bring the police.
Actually, Luring Jeremy over was Jan's suggestion. However .....
OK First theory - Jeremy hires a hitman;
So how would a hit man killing all of the family help Jeremy? He'd still need an alibi so would still need the phone call to distance himself from the crime scene. Surely the hit man would have to KNOW that Jeremy intended to call the police (otherwise there is no alibi). So why would he wait until they turned up? Just so he could dance at the window before miraculously escaping undetected? Had Jeremy not told him he was calling the police and as a result, the hit man ende up being caught - Jeremy would STILL be busted.
Someone lures Jeremy over to WHF in order to kill him too by telling him "Sheila has gone carzy, she's got the gun";
The reason this theory falls flat, is because the spoof caller couldn't know for sure if Jeremy would come to WHF or if he would call the police. Such a person would make themselves a sitting duck with a house full of bodies.