Hmm I can't say this will work but my suggestion.
When somebody kills you it says "<$OUTPUT()> has murdered you".
If you can find this string and manipulate the script so that the <$OUTPUT()> can be placed into a text document using the <$OUTPUT()> as the main header and then <$USERNAME> as the subheader=0.
When the document is opened via an NPC it will check for the header <$OUTPUT> (e.g. a username of mdoti), once it has found your username it will load the list of subheadings (other players) with the amount of times you've killed them.
This is just a theory so I can't say it will work but this is how I would do it if I was going to look into it.
Example
Code:
#IF
LoadValue D1 "PVPList.txt" %A1 %USERNAME
#ACT
INC D1 1
SaveValue D1 "PVPList.txt" %A1 %USERNAME
Sysmsg "<$OUTPUT(A1)> has murdered you <$OUTPUT(D1)> times"