You are obviously ignorant. If you know someone killed the kind, loving father you adored, was fighting and killing people of YOUR Nationality, and was deceiving all his friends while using their trust as an advantage to FIGHT AGAINST them...
Infatuation has nothing on that. At all. And lol, what if it was the other way around? What if Shirley killed Nunnaly by accident? Would Lelouch just look past it and give her his love?
If anything, Shirley is acting and has acted unusually nice towards Lelouch for the whole thing.