[RPG] What conditions activate the Great Weapon Master feat


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.

Best Answer

"One" is referencing the melee attack

What GWM does is give you the option to perform a single additional melee attack as a bonus action if you either roll a critical hit or reduce a target's HP to zero (either by killing them or simply getting them to 0 HP) while using a melee weapon.

There can be only one

It is important to note that you only ever get one single bonus action per turn. Therefore, you cannot kill a group of weak enemies since you are still limited by the standard action economy of having available, in a given turn, your Attack action and your bonus action.