If I were to generate an address and it happens to collide, legally speaking, would a transfer constitute theft? Let's say that somehow I generate a new address which happens to already be "taken". I realize the chances of address collision are extraordinarily low as per Is it possible to brute force bitcoin address creation in order to steal money?, but let's just say that it happens.

For argument's sake, let's say I land on 1FeexV6bAHb8ybZjqQMjJrcCrHGW9sb6uF (currently carrying a balance of about 80,000 BTC).

If I were to then sign a transfer to another address, would I be doing anything wrong or illegal? I think it'd be immoral, but does it constitute any legal wrongdoing?

I don't think this is quite like finding an ATM card and guessing the PIN, but more like walking into a Swiss bank, guessing a random account number and PIN, and walking out with whatever is in the safety deposit box, right? Is that "stealing" ?
IANAL.

What you describe is the act of taking something that isn't yours in full knowledge that somebody else invested effort and money to accumulate it. 

Just as with your Swiss bank example: The rightful owner would be losing the funds that you misappropriated. Taking someone's property without their consent is theft.

I feel that this is independent of the way by which you gained access to the property, but rather derived from the intent of your action.

Perhaps more ambiguous would be scenarios where somebody receives a transaction to a key that you also control, or when you stumble on a key that has a Bitcoin balance that hasn't been moved for decades and could reasonably assumed to have been lost by the rightful owner.