If the injury is the foreseeable outcome of the act, and the prospective defendant did not take reasonable steps to prevent the injury.
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We now allow highway traffic accident victims to sue tavern owners who serve people who they know have had too much to drink.
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The 31,000 firearms deaths that occur in the US every year are largely the foreseeable consequence of the acts of the board of directors of the NRA.  Of course they should be sued.