Is it possible to modify the linux assembler in order to modify what fuctions do? I was wondering if it were possible to modify the linux assembler in order to change the function of "-" to "+" and vice versa.
I was also wondering if this would affect the whole system making it collapse. I apologize if this question is not appropriate here. Please explain why if it is the case.

E.g:
2+2 = 0
2-2 = 4  

My objective is to see what applications this could have for a potential computer virus (I hope this is allowed here).
While this is trivially possible, the fundamental flaw in your idea is that assemblers are not interpreters. Python programs are run by the Python interpreter, Swapping - and + in the Python interpreter would likely crash all Python programs. But normal programs are executed directly by the CPU. The assembler is not involved in their execution.