Hi!

The HUD element was made with vscripting and it's part of this mutation, not a separate mod. Vscripts are fine as long as you are working on mutations or maps because they can have dedicated script files. However, if you modify the default script files you are going to have conflicts with all other mods using the same method (don't do it!).

L4D2 scripting language is Squirrel. Learning that is a rocky road if you don't have prior experience of other languages. If you do then it should be quite easy to pick up. Here are the steps if you really want to go for it:

1. Create the HUD element:
https://developer.valvesoftware.com/wiki/L4D2_EMS/Appendix:_HUD

2. Create table where to store kill counts:
https://developer.valvesoftware.com/wiki/Squirrel#Tables

3. Update the data table and HUD every time an infected is killed. This one should be connected to "OnGameEvent_zombie_death" event function. Basically you just validate that zombie death was caused by a survivor, increment the value in the data table and call a function which updates the HUD.

An infected counter mod for normal campaign and realism game modes would be very nice and with all players status on screen, something like this does and encourages players to improve their performances, if possible do so because the community appreciates it.