First create a variable called 'menkilled'

Next create a trigger which initiates only when the game begins which sets the variable to 0 (not even sure if its necesary but a habit)

i_set menkilled$ = 0

Next create another trigger called counter, this is where everything is done, fill in the parts which are stated here.

Conditions:

Entities,
Player: Player 1
State: Dead
Count: >
Value: menkilled$

Commands:

Set_i, menkilled$
op: +
Value: 1

Reinforcement,
Operation: score_add
Score: 1

Trigger,
counter