What I mean is, suppose we could somehow get a kilogram of antimatter and contain it safely. Now lets say we want to make a bomb using this kilogram, now, we have two ways, either store another kilogram of matter inside the bomb itself and let the matter and the antimatter touch each others when we want to bomb to detonate, or just expose the kilogram to the air and it will explode. But, my question here is simple, either of the previously mentioned ways will just allow the first particles touching each others to annihilate and sending the rest matter and antimatter in opposite ways making the reaction harder and slower to continue. I know eventually the whole kilogram will be annihilated, but it's all about reaction speed in explosives and that's the main difference between nuclear reactors and nuclear weapons. So now, is there a way to ensure that the matter and antimatter will completely annihilate each others with a high rate of reaction ?
You face exactly the same problem as the makers of the first explosives. Although these days we use explosives that react intramolecularly, the original explosives like gunpowder were made from a mixture of an oxidising agent (potassium nitrate) and a reducing agent (sulphur and charcoal). To get the gunpowder to go bang rather than fizz you had to mix the reagents extremely intimately so that the transport of the reacting molecules/atoms was faster than the explosion.
So basically you need an extremely intimate mixture of the matter and antimatter. That is going to make containment fun!
The obvious answer, to my mind, would be to build an antimatter bomb like a fission bomb. You have a shell of matter (probably the containment unit) around the antimatter which is crushed by some kind of explosive, perhaps a nuclear explosive, creating a rapidly contracting dense shell that can partially overcome the centrifugal tendencies introduced by the antimatter detonation.
Somehow, I don't believe the question of "quickly" needs to be applied here. The reason is simple. For a fission bomb, there is a need to maintain confinement in order for the nuclear reaction to continue - as density reduces, the chain reaction can extinguish.
On the other hand, for an "antimatter bomb", as the antimatter heats up, it will start to expand; in doing so, the fragments of antimatter will continue to encounter matter, and will continue to annihilate.
The best thing you can do to improve the "yield" of your imaginary device would be to create a mechanism that fragments your kilogram of matter and sends it in all directions. A single cubic meter of air contains about 1 kg of matter, so your device won't have to disperse very far to encounter plenty of matter to annihilate with. And given the available energy, it really doesn't matter whether all that energy gets released in one cubic meter, or 1000 cubic meters; and whether it happens in one microsecond, or ten seconds. Either way, most of the energy will escape in the form of radiation; the damage caused by the local heating of the air will be small. Your bomb will set fire to everything in a large radius because of the radiation, and that will be the main damage mechanism; this neither requires a fast explosion, not a "well contained" one.
Read "Angels and Demons" by Dan Brown if this kind of thing interests you.
What I mean is, suppose we could somehow get a kilogram of antimatter and contain it safely.
"contain it safely" is science fiction.
There is no way to keep antimatter in a matter environment safely except by the use of electric and magnetic fields, suspended in them .
Even suppose one can create ( another science fiction scenario which I will not consider now) a charged one kilogram mass of antimatter the devices needed to create the electric and magnetic fields to suspend it would be enormous and a barrier to manipulation.
The only manipulation of the antimatter mass could be done with electric and magnetic fields again, or laser light. Consider the devices needed to drill holes in the antimatter bulk so that , for example, matter streams could be aimed for more efficient annihilation. This will be a laboratory, not a transportable bomb.
For landing back to reality please read how antihydrogen, has been created and contained.