Because Lucy is love <3 , while MANkoto is an irritating asshole
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Actually the comparison between her and Kira kinda works, however she is certainly not intelligent enough to think about the bigger picture, and foresee that the result she could eventually manage to make, is only temporary.
I can also agree about disliking her, because killing anyone with such a method is a definite no-no, regardless of whatever noble intention she might have.