How to kill the emulators?

Find the location of the Android SDK

File -> New Projects Setup -> Default Project structure
Copy the address in the section of Android SDK location

set up adb

Input adb, if adb: command not found, you have to set up.
Open the terminal
Input echo $HOME
Create .bash_profile file by input touch .bash_profile
Open .bash_profile file by inputting open -e .bash_profile
In this file, enter export PATH=${PATH}:/Users/XXX/Library/Android/sdk/platform-tools:/Users/XXX/Library/Android/sdk/tools

XXX should be replaced by the proper name of your address.

Closing .bash_profile file will automatically save it.
Verify whether it's set up successfully by enter source .bash_profile
Enter adb, if there is no -bash: adb: command not found, then we are successful.

close the emulator!
enter adb emu kill to kill one emulator
if you have more than one emulator

enter adb devices to find out the serial numbers of the running emulators
adb -s <serial-number> emu kill to kill specific emulator
for example, adb -s emulator-5554 emu kill