There's new evidence that the death of Celeste Rivas was not accidental ... and it's something people didn't pick up on over the weekend.

Neo posted on Saturday ... that he's clamming up over the mystery surrounding Celeste's death. In the Insta post, he says ... "... at the end of the day this case isn't about me, it isn't about you, it's about getting justice for the victim and her family ..."

The word "victim" is critical ... our sources say the L.A. County Medical Examiner has had enormous trouble establishing cause of death, in large part because of the state Celeste's body was in when it was discovered in the trunk of D4vd's Tesla.

We do not know if the M.E. ultimately established cause of death ... but even if they did rule it a homicide, they had so much trouble it will almost certainly be a hotly contested issue when the case goes to trial.

Here's the thing ... if the defense argues she could have died from an accidental overdose, Neo could be a critical witness. Calling Celeste a "victim" does not suggest an accident -- it points to a deliberate act ... homicide.

TMZ broke the story ... Neo has broken with D4vd, calling him a "di*khead" and saying he has a lot more information about the case than has been reported.

Our sources say it's all but a done deal that D4vd will be indicted for the murder of Celeste Rivas. Our sources add that Neo is an important witness in her disappearance and how she ended up in the trunk of D4vd's car.

Neo has removed all his posts and has unfollowed D4vd.