systemd is like the service manager for your Centos and other modern Linux distributions (including Debian/Mint/Ubuntu) allows you to enable services, stop them, restart them, check their status and even reboot your system.
The key commands or arguments you will use with systemctl are the following:
list-units [PATTERN...] List loaded units
Enter the console as root
mysql -u root -p
Then kill the process (where 1234 = the thread ID)
It's also a good idea to use a tool like "mtop" to monitor which processes and queries are slow, killing them is a temporary fix while you try to isolate the actual problem.........