Is that really the golden rule? What about "the good of the many"?

Returning it to Blizzard directly is good for Blizzard. Returning your code is good for you. Realistically, it has little monetary value (all the "value" will be spent on lawyers arguing what the value is), so maybe it's only good for you because it's simpler for you.

What if your sense of morality is not in line with everyone else's sense, and I'm returning code to someone who will not do "the moral thing" afterwards? Does that make me immoral?

Point is, this is not an easy decision at all. It's not an easy answer at all. I'd be petrified and I believe anyone else who spends time to consider the implications would be too.

The golden rule is, like most other unilateral rules, an oversimplification. Of course, that's what makes it attractive — it's an effortless substitute for having to think through the complex and messy realities of any given situation. But let's not kid ourselves that something is automatically good because we would want that thing in Blizzard's situation, case closed.

so does "do unto others", if you follow it.

additionally you just argued that the same moral rules and rights that apply to you, a living breathing individual with an inner drive to enact right over wrong, can be transferred to an entire corporation, a legal construct blind to ethics unless forced by legal rules. which is kinda inhumane.

and, the golden rule is only a good moral yardstick if your choices in what you'd want to have others do unto you are moral in the first place. not a very nice thing to question, sorry, but your hypothetical example does feature you writing groundbreaking software but wanting to keep it closed source. depending on the software and how groundbreaking it is, that's an open question, very much up to discussion.

What if you said that after the copyright had expired? Would you still say it's immoral to release it against your wishes?