The masteries can be reverted back with right click and changed throughout the game. It's shown when hovered over the black section bar. But we will find ways to make it more easier to notice in the next update. It is to be changed by the player in-game game for different situational requirements and different playstyles. It's currently, not just completely meant to be a linear stat based progression.
Kills will be removed next update, gold will be awarded from killing enemies instead. Should fix this.