What the crap are you talking about? 100mA can stop your heart. Why would people go around being electrocuted by a car battery? I turn my car on with a key, not by shorting the leads with my body.
And yes, PSUs are transformers, at least functionally and they incorporate transformers as well as power switches. As I said they are complex creations and not to be made at home.
Yes, 100 mA can kill you. But, to make that current to flow thru your body, it needs to have a given potential (because, as you will know, our body is not a good conductor). Depending on a lot of factor (skin resistance, humidity, position of the electrodes,...) the voltage that can kill you starts from 50 VAC at the very least, and goes up to several hundreds. You can't even feel anything if the potential is only 12 VDC, disregarding the maximum current that it can generate.
Depends on which side of the switching circuit you decide to poke your fingers into. The 12V on the business end might be safe, but what you get from an unregulated discharge of the capacitors on the other end is a whole different story. Probably one with a sad ending.