As GM, you always have more options than you think, and you should always try to choose the one that's the most fun. My vote would be for the following:
4. This infuses the tarrasque with negative energy, creating an eldritch horror that threatens all of existence.
The rules don't say exactly where the tarrasque came from or how it works, but it's safe to assume that the creature is powered by some unlimited source of magic energy deep within its hulking form. The animate dead spell will corrupt this power source with negative energy, creating a threat to existence itself.
The most immediate effect will be the creation of an undead creature much more powerful than the original tarrasque, and certainly not under the control of the PC necromancer. The power of this creature will only grow over time as its power source becomes more corrupt. The creature will emit a negative energy aura that extends for miles, granting many permanent negative levels to creatures who fail their save and transforming most living creatures in the area to undead. As the creature's power grows, nightshades will begin to come forth from the negative energy plane to join with it, and it will begin to emit waves of negative energy of increasing radius. All natural healing of living creatures will cease, the dead will begin to rise from their graves, and material plane will begin to merge with the plane of negative energy.
It's up to the PCs to find out where the original tarrasque came from and how they can undo the damage that they have done. They might also have to deal with powerful planar beings who would use the abomination for their own ends, or who hope to defend the multiverse from the threat now posed by the material plane.
Temporary undead
A wizard with Spell Focus (Necromancy) can take the Skeleton Summoner feat, which adds a Human Skeleton (1 HD) to Summon Monster I's list and Human Skeleton Champion (3 HD) to Summon Monster III's list. It also allows you to apply the skeleton template to a summoned creature once per day.
The Zombie Skin Shield allows you to create a zombie for 5 minutes per day, as if you had created them. The price of 2,159 gp may be too much depending on your level though.
Permanent undead
A wizard with the Undead Master archetype can gain the control of undead creatures as if he was a cleric with the Command Undead feat, by spending a few uses of his school's ability. This will not create undead though, only control existing undead, but once controller, they are yours to command.
A tiefling with the right heritage (and some GM's discretion) could be allowed to raise 1 HD zombies/skeletons once per day as a spell like ability.
With Craft Wondrous Items, you could create a Death's Head Talisman. It has a requirement of being able to cast animate dead, which can be bypassed by increasing the spellcraft DC by 5, for a total DC of 15. The item's description says that the creation of the undead is part of the process to create the item, so no onyxes are necessary for that. The item should be affordable aswell by 2nd or 3rd level (625 gp).
If your GM allows 3rd party products, the Deep Magic has a spell called Raise Lesser Undead (2nd for cleric and wizards), which raises a single zombie or skeleton with HD equal to half your caster level (maximum 10th caster level, or 5 HD). It still costs an onyx of 25 gp per HD of the undead created, and you can only have one undead created by this spell under your control. "It's third party...", yes, it is, but it has paizo developers among the designers, including Jason Bulmahn (paizo's lead designer), if that gives the book any credit with your GM.
Best Answer
Speak with your DM about adding speak with dead to your spell list.
Adding speak with dead to your spell list would not be a big deal. This is affirmed by guidance from the Dungeon Master's Guide:
Obviously, it requires permission from the DM, but drawing on my experience as a DM and a player, I can foresee no problems arising from this particular change (your mileage may vary with other spells). Sure, it's technically a house rule, but it is a house rule that is explicitly encouraged in the published material with published guidance on making it work. Call it HRAW: house rules as written.
Speak with dead may still present an ethical dilemma for you.
Speak with dead states:
At best, speak with dead can tell you if, at the moment the creature died, they would have been willing to be reanimated. The spell does not allow you to communicate with the proper soul of the creature, and so you have no way of knowing if the creature's soul has since changed its mind and withdrawn consent. If you're exploring the ethics of consent with this character, this is something to consider. Thanks to Carcer for making this observation in comments.