Upload the "Killsound" file to C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf\custom 
Enable killsounds in the advanced section of TF2's in game setting browser
Included in this zip file are the manual download with only the sound file and the 
easy download. Copy the easy download into the directory you have Steam installed, or copy the 
"Killsound" file in it into your custom. you can also get the killsound with just the sound file.
For this, make a new folder in your custom named "killsound", inside of that file, put "sound", 
inside of that one, put "ui", and inside of that one, put killsound.wav, and then you should be good 
to go. Good luck!