Elemental priorities and ignore damage...

... You don't open the ROM in a disassembler to use ASM. You write .asm files and then apply them to the ROM using an assembly program, like xkas or asar. So, yes, you were definitively wrong.

Disassemblers (like DUDE, which Madsiur mentioned) are used to render the game code as readable assembly, but aren't intended for directly hacking the code. You can modify game code live with debugging emulators, but in my experience that's more useful for working with data rather than code. (I've used it primarily for experimenting with window sizes, personally, and occasionally with live-modifying SRAM values.)