Apps don't necessarily hog memory. If there is available memory unused, Android will load apps it thinks you may use in the future into memory to use that memory. That way, when you do launch those apps, they will come up faster. Apps that are loaded into memory that are unused would not be using any CPU.
If you try to manually manage the system resources yourself, you may end up confusing the Android OS and cause it to use system resources like memory inefficiently. It is best to just let Android manage the system resources.
If you are having some issues with your phone, perhaps you should let us know what exactly they are. If you are not having any issues, I recommend you just leave it alone and not try to kill or delete any processes or apps.