Wouldn't it be more ethical just to alter their life paths so they didn't meet their future spouses, or something?
Possibly, but she's not Asimov's Second Foundation. We don't know very much about how time travel works in this show, although I suspect it won't be terribly original in its rules: if that's the case, any attempts to change the past might be like small wounds which the timeline corrects as soon as and as thoroughly as it can. The parents and grandparents would meet anyways, just under different circumstances.
Besides, the show's gravest problem (and paradoxically most interesting theme) is that she's apparently a good person in the service of bad masters, while the terrorists are horrible people pursuing a good cause. Even with that set-up, Keira has happily flounced about cleaning out bank accounts and calling in police when she needs them: the terrorists killed some of those people, but Keira arranged for the victims to be there in the first place.