I have phantom electricity
Certainly sounds like you have a wire/outlet that is connected to two different circuits, given your symptoms.

First - when dealing with something whacky like this, just shut off the main breaker if you need to, say, cut it. If it's still hot with the main off, you have some really interesting wiring going on. And if, for some reason that should never really occur, you have a need to cut romex live, use insulated handle wirecutters and only cut one side at a time, then cut the ground wire last from the neutral side, rather than shorting out the wires in the cable with the cutters. That was not a very good or safe approach to the problem (you could have just left the thing alone until it was sorted out - cutting it does not seem to have helped in any way, and I don't see any way it WOULD help.)

Second, see what happens if you turn off BOTH the "breaker on the main" and the "breaker on the subpanel" that you identified as being associated with it. NOW is it dead?

Then, go looking for where someone connected the two circuits together. Or, call an electrician to have a look - the life you save may be your own.