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The conjunction li prefixed to the verb yahmilu ("they shall carry") has here obviously - as pointed out by Razi - the function of what the grammarians call a lam al-'aqibah, indicating no more than a causal sequence ('aqibah); it may be appropriately rendered by means of the conjunctive particle "and" or - as in this context- by the adverb "hence".
Lit., "those whom they are leading astray without knowledge" - i.e., without knowledge or understanding on the latters' part (Zamakhshari).
Their responsibility or crime is twofold: (1) that they rejected Allah's Message, and (2) that they misled others. Their Penalty will also be double. In vi. 164, we are told that "no bearer of burdens can bear the burden of another". This is against the doctrine of vicarious atonement. Every man is responsible for his own sins; but the sin of misleading others is a sin of the misleader himself, and he must suffer the penalty for that also, without relieving those misled, of their responsibility.