Killing the flow, slow venom
Sinking his fangs into the flow and injecting his venom.
The toxin in venom slowly shuts down the nervous system and stops the heart and lungs (resulting in death).
Flow is the rhythm of a rapper rhymes, and occasionally the delivery as well. Eminem is saying that he is killing the flow — as in doing it very well — so that it’s venom, essentially so it’ll pervade every part of you. And for his opponents, his rhymes are so good they will debilitate them.
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