but when we split the atom the result was Hiroshima
Nope, when we first split the atom, it was 1917. It wasn't until people discovered the possibility of Nuclear Chain Reactions (where Atoms produce the right components to continue the Nuclear Fission, or splitting), back in 1933, that it was even thought about as being a fuel source.
A Nuclear Bomb only explodes because it is a chain reaction that is set up to run on until a critical point. And the mass inside that bomb contains well over billions of atoms. It's when the many many many atoms all splitting that you get the explosion.
Now, in the LHC, atoms are collided together using intense Electromagnetic fields which accelerate them to near light speed. It takes an awful lot of energy to accelerate anything to near light speed. (In fact, it takes a nearly infinite amount of energy to get to light speed, but that's another story).
Once those accelerated atoms collide, the majority of the energy is lost. It's that energy loss that gives us the particles that we're looking for. So unless someone's got a way of shrinking all the accelerators into a bomb shaped device, there's no chance of an explosion.
And if you're worried about it becoming a massive implosion (i.e. a black hole) then that's not going to happen either.
The energy used at the LHC is too low to produce a black hole, according to the Standard Model of particle physics. Extensions to that model predict that micro black holes may form. But micro black holes will vanish because all black holes release Hawking Radiation and micro holes can't absorb enough energy to keep themselves stable.
The LHC is awe inspiring, yes. But it's not something to be terrified of. Like any other tool of humanity, it's how it's used that matters.
(and hopefully this post makes sense, as I was going to throw numbers in and then decided that might not help)