Knowing Allah is Merciful does not permit us to just go about doing sins and not even saying we are sorry for them and seeking his pardon. We would not accept this from another human being. Imagine if we are merciful but someone keeps trampling our rights and treating us bad then they never apologized. We would not accept this from them and be upset with them. We must do tauba and ask Allah’s forgiveness and not feel arrogance that we deserve Allah’s mercy.