What is the ruling on one who knew of a killing and overlooked it, thinking it was a joke?

The condition for denouncing evil is that it must be manifest and currently occurring, except for evils that cannot be rectified, such as death and fornication, in which case it is obligatory to spy and search them out to prevent their occurrence. If the perpetrator of an evil will not desist unless reported to the ruler, then it is obligatory to report him. If a person knows that murder is likely to occur, it is obligatory for him to strive to prevent this evil, by warning or other means, according to his ability, because it is a communal obligation that becomes an individual obligation upon him if it is highly probable that others will not fulfill it.