I've cut almost 10 keggles open with a handheld plasma torch. works GREAT! Just be sure to fill the keg with water first or the gasses will stain the sidewalls and it is a beast to clean up.
This is the end result of my method. I used the rim of the keg as a guide and also rested the tail end of the handle of the torch on top of the rim. This offers lateral support from the rim as a circular guide, as well as vertical support from the back end of the torch. It leaves a nice WIDE opening on each keggle. Notice here I am cutting the bottoms out of the kegs because I bottom drain all my vessels.
Also, WEAR GLOVES! THICK GLOVES! With water in the keg, the torch will start blowing that water (hot water by now) up on your hands and if you are wearing cheap thin work gloves you may get scalded.