Oh, dear. The GM has informed me that taking 'Reap the Whirlwind' will most likely end up with my character killing a party member.
Seems to me he's taking the aspect a bit to literal... You could easily change the aspect to "Sow the wind, reap the Storm". Thats basically the same, just not the common form the quote takes in english.
(It's closer to the german version, which is "Wer den Wind sät, wird den Sturm ernten". In english "Who sows the wind, will reap the storm")That would be without the element of "whirl" which i suppose is what made your GM say that there'd be collateral damage.
The following interpretation could then apply: that when people do something bad, they can expect you to reward them kind, just a lot harder.
Not that this interpretation needed to be explained here, i bet.*g*So as long as your party members don't get rowdy... there'd be no reason to compel you to kill them.
I could also be totally off trying to deconstruct your GM's thinking though *g*