When a hero with the Necromancy skill kills a monster on his turn, may the hero immediately resurrect the monster AND activate the monster? For example, on his turn, Battlemage Jaes kills a beastman, "necros" him and then orders him to attack another monster. Is this allowable or must the hero wait until his following turn to activate the necroed beastman?

The necro'd monster acts after the hero, so it would be in play on his side the same turn it was killed. I don't see any reason he shouldn't be able to use it immediately.