hacking what'sapp with malware! no way if your target use pc or desktop for her WhatsApp then spyware is good malware & spyware are two different things, also writing any bad code in many high level language like C#,python,java,vb,kotlin,etc are not good option because av program can easily decompile her & detect the signature of your bad code.
make sure you work with some low level like C,C++, Rust will i strongly recommend to use C or Rust the king of the hardware & everything in your hand
fell free to ask