Does the word "slaughter" has stronger nuance than "kill" in this context? Because I thought it sounds more aggressive.
Yes, it definitely is a stronger and more aggressive word. In this context it means to kill in a cruel or violent way, usually in great numbers. This is why it’s more generally used to refer to events such as the butchering of animals or killing many people in a battle or war. Ex: The soldiers slaughtered their enemies. Ex: The victims were slaughtered in large numbers.