How to rip out inground pool's steel walls?
I'd personally be surprised if they were steel, but a magnet should be handy for verifying that they are not (or are.) 

If you are in full destructo-mindset and not going to change your mind over the winter, drilling hole(s) and looking at what comes out of the drilled hole (and what you see looking in the drilled hole) can also be very diagnostic.

If, as seems far more likely to me, the pool is gunite or fiberglass, all you need to do is break up the top rim deep enough that it won't affect future uses (lawn, garden, etc) of that area, and fill it in - unless you are expanding the house into the area. 

8 foot deep holes/trenches are distinctly hazardous to be in - if the dirt collapses, it can kill you. Either use digging equipment that keeps you out of the hole, or dig it very wide/sloped.

If it's actually steel, you'll want a 9" angle grinder to make short work of cutting it up, or a 4" if you find a 9" too much to handle. The 9" is faster but requires more effort to control. Investigate tool rental, and get double eye protection (facemask and goggles) and a cheap set of welding leathers so you don't set your clothes on fire. Grinder sparks hurt, and set fires if there's any fuel for them to hide in.