[Black Rose]

I have a couple comments still.

    precache_sound("sound/headshot.wav");

I'm not sure here but I think sound/ is not required when precaching a sound file through precache_sound(). It will assume the sound folder, so this will print as "sound/sound/headshot.wav". At least I get this error: "Warning: Unable to open sound/sound/headshot.wav for transfer"

register_event("deathmsg", "hook_death" ,"a");

Events are case-sensitive. "DeathMsg" would be correct. You could also filter away killer being 0 and the wepon here. by adding this to the end of that function

"1!0", "4=knife"

This basically means "parameter 1 is not equal to 0" and "parameter 4 is equal to knife".
You could also add "3=1" to make it only for headshots.
Here's a full example that will only get called if the killer is not the server/map/worldspawn (suicide), it's a headshot and it's done with a knife:

register_event("DeathMsg", "hook_death" ,"a", "1!0", "3=1", "4=knife");

To make sure the killer is a player and not an entity you should check if the player is in the range of 1 and maxplayers. Create a global variable named g_maxplayers and retrieve get_maxplayers() on plugin_init(). Assign that value to the global variable. In the DeathMsg event you need to check if ( killer <= g_maxplayers ) before running any player based functions (like get_user_name(), get_user_team() or get_user_frags()) on that id.

I can see that it's not done so I will stop there for now and let you try for yourself a bit. Just come back if you have more problems. But at least register a cvar for the getmoney pointer because you will get runtime errors if trying to retrieve a value on a cvar pointer that doesn't exist.