There are two types of Bad Ends: Story and Relationship.

Bad Story Ends: Happen when you choose answers that are not optimal to helping the character who’s route you are in achieve their personal goals.
Bad Relationship Ends: Happen when you either don’t participate in enough chats or don’t participate in chats at all.
This post contains all bad ends that are contained inside the game and I will also add in bad ends for any new DLCs that come out as well.

Prologue Bad End
You can Bad End in the prologue (and meet Unknown) by repeatedly lying about not seeing a place for a password. You will then be kicked into Visual Novel Mode where you will meet Unknown. No matter what order you choose here, you will always Bad End.

Casual Story 5th Day Bad Story End
Play Casual Story from 1st Day, not focusing on any character [walkthrough here]. If you do choose to get into chats and you are in a chat with Jumin or 707, choose their answers over the other three characters. Otherwise choose Nothing answers unless the game forces you to choose between two characters in Casual Story. Try to do your best to flip back and forth if the game makes you choose an answer for Zen, Yoosung, or Jaehee. On the 4th Day Branch, the plaque under the 5th Day will be empty if you have successfully reached the Bad Story End.
https://crgsoft.com/mystic-messenger-endings-how-to-get-them-all/
Deep Story 5th Day Bad Story End
Play Deep Story from 1st Day, not focusing on any character [walkthrough here]. If you are in a chat with Jumin or 707, choose anyone else’s answers over their answers. Otherwise choose Nothing answers unless the game forces you to choose. Try to do your best to flip back and forth if the game makes you choose an answer for Jumin or 707. On the 4th Day Branch, the plaque under the 5th Day will be empty if you have successfully reached the Bad Story End.

5th Day Bad Relationship End
This ending happens when you either don’t get into any chats at all or don’t get into enough chats. It is the same ending on both days.

Christmas DLC Bad End
Play Christmas Eve concentrating on getting into someone’s route and making sure to spread out everyone else’s. On Christmas Day after 00:01 – Ungrateful Christmas. You will begin getting texts from Unknown. To get this end, I answered him this way:

Yes, I want to know who you are. (Triggered by participating in 00:01 – “Ungrateful Christmas”)
I’ll send you the address for the event [Send address] (Triggered by participating in 08:43 – “Yoosung and C&R”? Need confirmation)
It ends at 9. I’ll be waiting. You said you’d come find me soon. (Triggered by participating in 16:07 – “All because of [Name]”? Need confirmation)
Please note you HAVE to answer all three texts as listed above. If you don’t, the interaction will stop and you won’t get the bad end.

Continue to play Christmas Day and answer Unknown’s text as soon as they come in. I did not really pay attention to whom I was answering for EXCEPT on 00:01 – Ungrateful Christmas, where I chose:

Selection 2 – Does that mean you have to practice all over again? That’s too much.
Selection 6 – That’s right. I hope they treat their actors with more with care.
At 18:10, you will get sorted into a route. Here, once again, I did not really pay attention to whom I was answering. The game then branches after the 19:23 chat and you will find your route chats have now changed. Now you will see the chat “Reached the goal.” Congrats! You are in the bad ending! Enjoy the surprise! >:D

There are two CGs for getting this ending that will fill into the Common section.

(Further spoilers, please only read after you have completed this ending)
And before anyone comes in here and complains about this being labelled a bad ending, I’m going to tell you why it is.

After the reveal happens, we are no longer able to choose reactions from the MC. Unlike in other bad ends, we cannot choose to have her egg this ending on and even her reaction is trembling, but we don’t know if it’s with anticipation or fear.
The RFA group, upon figuring out that something is VERY off, all begin to panic in the chat. Even Seven, who tries his best to keep calm begins to crack from the realization that something bad probably happened.
You have to remember, from playing the main game, that Unknown only cares about the destruction of the RFA/Seven. Kidnapping the MC and holding her as his doll to possibly be never seen or heard from again will probably destroy Seven as it was his job to make sure she was safe in the apartment which, in turn, will destroy the RFA.
Seven also does not know the real story behind what is happening. He can still hope the other person in his stories is happy because he doesn’t know what is really going on at this point.
All in all, a bad ending.

Christmas DLC Bad Relationship End
This route is pretty easy. Don’t get into any chats. Once you see chat at 18:10 – Reached the goal, you are in the ending. If you want to get into the chats for hearts you can or you can choose to continue to miss chats.

There are choices to say if you are going or not to the event. I chose to keep the MC saying she was not going.

This end will also happen if you don’t get into enough chats.

There is no CG for getting this ending.

[NEW] Another Story Prologue Bad End
You can Bad End in the prologue by repeatedly showing no interest in playing the game and not answering Ray when he calls. You will then time skip to a few days later in a cafe (that is the BG during Jaehee’s Good End). Continue to not answer the calls. When he tells you to wait for him at the cafe, report him to the police. The fact that this is a Bad End is kinda funny to me because there is really nothing bad about it other than you never get to meet the RFA, which is probably for the best…