If you already know the essentials of this map (what's in the map guide discussion), you're ready to achieve the alternative ending. So:

1.-ACTIVATE HARD MODE: This ending can only be obtained if you play in hard mode.
2.-ACTIVATE THE SECRET COMPUTER: Go to the computer in the following image and press the interaction button. No text appears on the screen, hence the secret!
https://imgur.com/iLO62TL
3.-COLLECT THE KEYCARD: Find the keycard located in the hallway before the parking lot, as shown in the following image.
https://imgur.com/AnN4Iwy
4.-OPEN THE SAFES: There are 3 safes in the map. Once you collect the keycard, you can open them and collect the capsules to assemble the nuclear bomb. Here are their locations:
https://imgur.com/QzLuSbv
https://imgur.com/SwglbXZ
https://imgur.com/9v7IEkn
5.-CHARGE THE NUCLEAR BOMB WITH SOULS: Go up the parking lot until you find the nuclear bomb. You must first activate it to start collecting souls. This will spawn Panzers.
6.-FINISH ASSEMBLING THE NUKE: On the table at the top of the parking, you can assemble the nuke with the capsules you collected from the safes.
https://imgur.com/I8IDUfS
7.-ACTIVATE THE NUCLEAR BOMB: After assembling it, you can activate the bomb. Survive during the countdown while bosses spawn. When the countdown concludes, the bomb will explode, ending the game. This is the alternative ending!