If you have reserved the under seat storage space for the middle seat, you should certainly raise a fuss with a FA. If you have not, it is not "your" space and the early bird, e.g., the other guy got the worm.

If your bag won't fit in the proper space, e.g. seat in front of you and there is space under an adjoining empty seat, it is between you and AA as to whether you may use that space. The other passenger has no say.

Having said this, there is such a thing as courtesy and making clear what you are doing before you do it. And having said this, there are those shoes worn by the assassin the "From Russia With Love" which contain a knife blade which extends from the toe and might be useful !