Almost certainly not. As in, "you'll have more chance of winning the Lottery than you will of killing them by this method".
Antimatter annihilation of a single atom - we'll be good here and say one with a hefty nucleus like, say, iron - releases
$$\left(2\ \mathrm{atoms}\right) \times \left(\frac{55.8452\ \mathrm{g}}{1\ \mathrm{mol}}\right) \times \left(\frac{1\ \mathrm{kg}}{1000\ \mathrm{g}}\right) \times \left(\frac{1\ \mathrm{mol}}{6.022 \times 10^{23}\ \mathrm{atoms}}\right) \times c^2 \approx 1.67 \times 10^{-8}\ \mathrm{J}$$
which is 16.7 nanojoules, or over 100 GeV, of energy. (The "2 atoms" factor is because you need a second atom's worth in equivalent - not necessarily in the form of a literal single atom - of ordinary matter to complete the annihilation.) The release of this will likely not be all at once, but rather will basically consist of the heavy anti-iron atom, upon teleportation to the brain center, annihilating with some lighter atom which will cause it to explode catastrophically into a shower of lighter particles and anti-particles as well as VERY hard (100 MeV+) gamma rays for the anti-nucleon annihilations, and these anti-particles will also collide with and cause similar explosions of the atoms they encounter elsewhere, producing even more showers of tertiary, quaternary, etc. ionizing particles. Essentially it's a demolition derby on an atomic scale with billions of bits of high-energy matter flying around and knock apart everything in their wake - DNA, proteins, and more. Keep in mind that a chemical bond has energy only on the order of 1 eV, so this is enough to break on the order of 100 billion chemical bonds.
Now that sounds rather extreme. But there's two things to keep in mind here: Even a single cell, if we for simplicity [and wrongly] treat it as a sphere of water 10 µm in diameter, contains about 17 trillion molecules and thus 34 trillion chemical bonds. Effectively there's only enough energy to break about 0.3% of them. Granted, that could be considerably destructive to that single cell, and thus you might expect we could at least kill one neuron with this (you cannot turn a neuron to cancer, because they cannot divide, though if you get something like a glial cell, then it's possible in theory, and this is a real and actually common type of brain tumor, called a glioma). However, that assumes all the particles are absorbed in the neuron, and that will almost surely not be the case, because that would mean total absorption within 5 micrometers assuming it appears dead center, and these forms of radiation are far more penetrating. The result is maybe you might break a few thousand or million of bonds all over the entire brain - something with maybe over $10^{24}$ atoms in it. That will be virtually unnoticeable.
Which is what our second point is. The 100 GeV of energy released here corresponds to about a thousand typical 1 MeV particles of the type that naturally exist in background radiation, not taking into account the possibly increased penetration of some of the highest-energy products which will make it even less damaging(*). As a dose to the brain tissue itself, it corresponds to (assuming it like gamma, which will actually not, again, be right, but we just want the order of magnitude, and using 1.5 kg for the mass of a brain) around 10 nanosieverts (nSv) of dose. The average background exposure in the United States is 3.1 millisieverts (mSv) per year (cite: https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/bio-effects-radiation.html) or about 99 nSv/Ms. Thus your brain is dosed with about this much about every 0.1 Ms, or 100 ks, or a bit over a day (86.4 ks). In effect, you get all of an extra day's and change worth of normal background dosage for this stint. Very unlikely to kill, and impossible to kill "instantly". In fact such ultra-low doses may even have a protective, and not harmful, effect due to possible radiation hormesis (not sure what the evidence on this is as of now).
Nonetheless, there is a potentially useful lateral angle to this that might be worth considering, and that's that if people generally have a fear of things like "antimatter" that they've seen in movies and don't necessarily understand very well except that they make things go "boom", such a thing could be a useful psychological control tactic on at least some of the population. If you want to make the threat credible, I'd suggest instead having some kind of device in the brain that creates a small artificial aneurysm. A burst aneurysm can kill very fast, and if the device can also self-destruct so as not to leave residue, could look like a "natural" event to an unsophisticated autopsier. Such a thing might work by, for example, being placed near a suitable blood vessel and then, upon triggering, would start a release of some kind of chemicals that partially break down the vessel wall, weakening it and thus allowing for a swelling or hernia of blood (the aneurysm) to form, that then bursts and causes massive brain damage. All the better since you can control the placement in the brain to target the areas most likely to cause death or at least major disability.
(*) You might think highly-penetrating radiation is "worse" than lower-penetrating, e.g. gamma is "worse" than alpha, but this is only with regard to the fact that an external source of alpha is "better" in that it can only burn the skin, but gamma, due to being penetrating, can "burn" all tissues through the full thickness of the body uniformly, leading to radiation poisoning, essentially a "systemic radiation burn". But that's only for an external source, with the skin blocking. In fact, if the source is ingested, alpha particles are much worse, because they have much more ionizing punch per particle. Effectively you're now comparing them both on a fair playing field as full-body irradiators, and the gammas are considerably less damaging due to the fact that greater penetration means less chance of interaction. This is part of why that polonium-210, and not, say, cobalt-60 [a strong, and relatively "pure", gamma emitter, and much easier (and cheaper!) to get ahold of], was used to assassinate the late Russian defector Alexander Litvinenko a few hundred megaseconds ago. The needed lethal dose was much less due in part to this fact.
