The dative of agent
A gerundive construction will use a dative agent, and not the usual ablative associated with passive agents.
Therefore we find:
Spartacus militi necandus est
Spartacus must
be killed by a soldier.
Where in the perfect tense the
sentence would be: Spartacus a milite necatus est
Spartacus was
killed by a soldier.
No preposition before the dative is necessary.
