Hi everyone, I'm going to copy/paste a lot of info. from the forums here regarding how the game decides what ending you'll get.

According to GameFAQs, endings aren't based entirely on deaths, etc. The game has a hidden score. If you take damage from weak or strong enemies, you get hit by bosses or Penguin, or you die, it will affect your score. Here is how the game tracks points according to the guide:

"In this game, you'll find that your health - specifically how often you're hit and how many times you die - determine the ending. Taking hits and dying adds points to a counter:

• a hit from basic enemies: +8
• a hit from middle grade enemies: +12, +16, +18 or +24
• a hit from bosses (Penguin excluded): +48
• a hit from Penguin (both boss fights): +80
• dying: +1,024 (overwhelmingly the greater influence).

Basically, outside of dying, the points added to the counter are proportional to damage received. When you finish the final stage, depending on the values of this counter, you will get a different ending. The estimates in regards to number of deaths are merely estimates, as taking a certain number of hits is still equivalent to a loss of life."

More info. here: https://gamefaqs.gamespot.com/nes/587118-batman-returns/cheats

And as for endings:

Hidden score is less than 5,376 = Best Ending (Awesome! You are the hero of all heroes!)

Hidden score is between 5,376 and 9,984 = Good Ending (Congratulations, Batman! You've saved Gotham City.)

Hidden score is between 9.984 and 14,848 = Bad Ending (Your skill has improved, but you're not a real hero yet.)

Hidden score is more than 14,848 = Worst Ending (You took too much time to get here. Try getting here sooner.)

With all of this said, it is possible to get the Good Ending despite dying. In fact, I did it while prepping this game for NES Odyssey 2024. I'm going to link a video of my stream archive; it shows me dying in Stage 5, then I keep playing, and eventually I still get the Good Ending. I played on regular cartridge + top loader NES.

Turns out dying at least once is still okay (depends on that hidden score). Obviously, dying at all is slow and this game is still fairly punishing. However, you can still get a Good Ending even if you struggle!

Hope this all makes sense. Good luck on your runs!