Warlocks make fine necromancers. They’re not the most minion-manipulating of approaches to take, but warlocks inflict necrotic damage, curse enemies, manipulate spirits and even draw immediate power from an enemy’s death. They have flavorful implements, and leather armor works quite nicely with Guildsman fashion. The “warlock’s curse” would repreesnt the connection between a necromancer and the life spark of one’s enemy. The Disciples of the Abyss likely feature a number of necromancers who are warlocks by the mechanics.
The infernal pact involves a lot of fire damage. However, the pact boon and the default Life-Stealer paragon path are incredibly necromantic in fashion. A strong choice, particularly if you can convert some of the fire damage type.
Strongly thematic, and with plenty of cold and fear. The doomsayer paragon path is very appropriate. Probably the most appropriate of the basic three.
Potentially the weakest, thematically, though the pact boon can be translated into a quick step through the Underworld.
Another strong contender, with rather little reskinning necessary.
This is the strongest pact for the concept of the necromancer who has some mastery over guardian ghosts. Each vestige, of course, represents a ghost that has been bound to the necromancer in question and assists him. This can be a trigger for some excellent roleplaying, if desired — the warlock gains new vestiges in play as he rises in level. Are they the ghosts of enemies he and his compatriots have defeated? The answer “yes” is so entertaining I personally couldn’t resist exploring it.