・・・。
Even so… there's no need to go and slap me.
Literal
Even-so... not-even slap [nominalizer-こと] not-exist [tag-question-じゃないか].
だからって is the casual contraction of だからと言って ('just because X, that doesn't mean...'), a soft pushback. なにも here isn't 'nothing' but the emphatic adverb 'not even / it's not as if' — 'it's not as if you had to slap me.' The fixed ことない pattern means 'there's no need to X,' and sentence-final じゃないか ('isn't it the case that') frames it as a rhetorical appeal. Stacking these three rhetorical softeners captures a whiny, slightly hurt protest tone hard to render literally.