“Soul for a soul” node of “Reaper form” skill does not increase damage. There is no difference with 3/4 node points (+60%) or 1/4 (+20%) in “damage stats”.
In comparison, the node “Reaper’s curse” properly adds damage. 4/4 (+100%) and 3/4 (+75%) I can see the difference of 25% in “damage stats” tab.