You just didn't gain many charges for killing the weak. Fact is, you probably gained at least one charge for this roleplay.

You may have dropped your ratio by doing it, but that's to be expected. Otherwise you could just keep multi-killing the same pk-weak guy and gain charges as if you were going after victims who represented the full spectrum of possible charge gains.

You killing them isn't wrong, per se, but it not being reflected in your charge ratio would be. Your rp was to kill people for certain kinds of behavior, whether they were strong or weak. That should not yield as high a charge ratio as someone whose rp is to kill only the strongest. At the end of the day, you probably come out with more charges overall, just a lower ratio per kill.