I think Aria doesn't want to kill Shiro, at least not until she found a way to replace her permanently, that is why she hasn't killed her directly. Also, Aria probably thinks Shiro is unkillable anyways, she would just reincarnate again in her opinion.
wait how can aria kill (my assumptions) the administrator? and why she scared by creation? I assume until now no creation first born were ever born in all previous garden but the people of creation side appear from other dimensions and destroying everything
