How do I kill process in UNIX using sudo and pgrep?

Hi:

I'm pretty new to the world of UNIX.
I'm a windows guy new to all of this.

I'm trying to identify and kill processes on my box.

sudo pgrep -f cassandra
----------------------------------
<returns the associated process IDs (good)>
14215
28140


So, to kill all the "cassandra" processes on the system:
--------------------------------------------------------------------------
sudo grep -f cassandra | xargs kill -9


I get the following results:
-----------------------------------
kill 14166: No such process
kill 28140: Operation not permitted


What am I doing wrong here?

Thanks,
JB