You might have to convert the Intel hex file to "raw" hex before using any of these tools. You could decompile to C, Java, or BASIC without knowing the original language used to write the program. Again since there are no variable names in machine code, You might get variable names like "Variable1" and "Variable2" instead of original-meaningful names like "RedLED" or "StartTime". There's more than one way to make a loop and the decompiler might choose a different method from what the original programmer used. An analogy would be if you gave a homework assignment to write a program that does some particular thing, every student's program would be slightly different although they all do the same thing. That means you can't get back the exact-original C/C++ program, but you can get one that functions the same when re-complied. There is NOT a one-to-one relationship between C/C++ and machine language. But, I have no idea if you can find one for the AVR/Arduino. There are also decompilers for converting machine code to C or C++. And, I don't know if there's a disassembler for the AVR or if you'd have to write it yourself or do it by hand. There are no variables, labels, or comments in machine code, so variables & labels will be simply replaced by memory addresses and there will be no comments in the disassembled code. That is, every assembly language instruction has a corresponding machine language instruction so it's fairly trivial to disassemble the machine language and convert it to assembly language. There is a one-to-one relationship between assembly language and machine language (the hex file). So, if you find a readable string you can read it, or you can change it with the hex editor. You can open any file in a Hex Editor and the Hex Editor will show the ASCII character for any values that can be converted to text (including values that don't represent text). In other words, the hex code is meaningless unless you know what the machine language is. C/C++ is portable (with some limitations) so you can compile a C/C++ program to run on an Arduino, PC, or Mac, but the machine language hex code only runs on the machine it's intended for.
(Or, they steal the source code or simply copy the hex code.) Realistically, when people do "reverse engineering", they usually figure-out what the code is doing and they start-over and write a new program that does he same thing.
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