An effect can be tolerated without being willed by its agent; for instance, a mother's exhaustion from tending her sick child. A bad effect is not imputable if it was not willed either as an end or as a means of an action, e.g., a death a person incurs in aiding someone in danger. For a bad effect to be imputable it must be foreseeable and the agent must have the possibility of avoiding it, as in the case of manslaughter caused by a drunken driver.
Source: Catechism of the Catholic Church (Vatican.va)