How many people did Nucky kill in Boardwalk Empire?

The fatality rate on Boardwalk Empire is so high, it's pretty easy to lose count. But if I recall correctly, you've only missed one other instance: The season three episode Two Imposters, where Nucky kills three of Rosetti's men who come to assassinate him and then another one of his men when the truck he's hiding in is stopped.

This brings the number of people we actually see Nucky kill to 6. However, the 4 kills in Two Imposters were pretty much in self defense and only the 2 kills you've mentioned were cold blooded executions. This number might seem low in such a violent show (and profession) but fits Nucky Thompson's methods, as Wikipedia states:

Throughout the series he is portrayed as a Machiavellian politician who makes his henchmen do the dirty work, while showing a more humane side to his friends and family. However, by the end of season 2 he is shown becoming more ruthless in order to compete in the violent bootlegging business.