So if, for example, a crime happens with 10 witness, and all 10 witness start dying one by one, will you simply say that there is no evidence the crime and forget about it?
In this case, there may not be any bulletproof evidence, but everything around the case that could've gone wrong has gone wrong, to the point where it's no longer statistically plausible to be a mistake. Everything in the system had redundancy yet every part of the system "failed".
There were two guards, yet they were both "asleep". They were supposed to check him every 30m yet they didn't check him for hours exactly when he killed himself. He had attempted suicide yet he wasn't on suicide watch. His cell mate was taken away. All the footage and backup were lost.
I guess the closest evidence so far is that the autopsy shows more homicide than suicide, but that isn't much.
Yes, it is indeed a conspiracy theory at this point, but unlike what the internet will have you believe, "conspiracy" != "crackpot theory".