kill -9 is like rm -rf. Sometimes you need it, but it's the path of maximum destruction. In the case of kill, signal 9 (SIGKILL) is a signal that processes cannot trap, so they immediately end without any chance of cleanup or saving state. Unless you really don't care about data the process may have been working with, it's usually best to try a plain kill (sending SIGTERM) first.
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kill -9 is like rm -rf. Sometimes you need it, but it's the path of maximum destruction. In the case of kill, signal 9 (SIGKILL) is a signal that processes cannot trap, so they immediately end without any chance of cleanup or saving state. Unless you really don't care about data the process may have been working with, it's usually best to try a plain kill (sending SIGTERM) first.