Kill Counters is an addon for keeping track of player killing blows for each of your abilities.
Any ability that you use to get a killing blow on an enemy player will start a counter and display on the spell tooltip both on action bars and in the Spell Book.
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/killcount
Shows the sum of all Killing Blows with all abilities -
/killcount detailed
Shows your total Killing Blows for each ability as well as the total sum