Unwanted Background and Auto-start Apps
Android doesn’t manage processes like Windows does. Properly behaved apps running in the background aren’t actually doing anything, they’re just remaining in memory and using no CPU or other resources. When you access them again, they’ll quickly open, as they’re waiting in memory for you to return.  If they were removed from memory, they would take longer to reopen as their data would have to be transferred from system storage back into RAM, in this way, a task killer can actually slow things down.

Android is made to manage apps automatically and intelligently for better performance! No need to manually stop them, they will be started again and again.

You should know that some apps needs their processes and/or services to run in the background! 

Ex: Facebook needs some services and processes to fetch new messages or notifications. 

So process killers will only drain the battery and consume storage.

You may need to read this article on why you shouldn't stop a background process using task killers.