Part of the Great Weapon Master feat (PHB, p. 167) says:
On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.
I quiet don't understand what is meant by one, the critical hit or the melee weapon? First I thought, if I kill a creature with one hit, I get a bonus action. Then a friend told me, one means the critical hit and the information
...or reduce a creature to 0 hit points with one...
is redundant, since no matter if the creature dies or not, as long as it's a critical hit (but only then) I get a bonus action. Also, if my first assumption is true, and a one hit kill as well as a critical hit gets me a bonus action, can I get another bonus action from this, if the prerequisites are met, and so on.
The idea by choosing GWM, besides all the other great opportunities this feat grants, was the chance of killing a whole group of weak enemies with my raging barbarian.