not like the LE has a video of him killing all the victims or a confession from the defendant that he did it

Are videos of the murder being committed coupled with a confession common in murder trials?

 I am saying if such evidence was present and people still believed that the defendant is innocent

It looked like you are saying that video of murder and confession is required for it not be unreasonable to think the accused is guilty, and that short of such evidence there may be doubt. A lot of people think he is guilty based on his DNA on a sheath for a large fixed blade knife under a victim killed by a large fixed blade knife; videos in at least 21 locations that morning all consistent in time, place and direction with travel between BK's home and the scene; synchronous movement of his phone with the car before and shortly after the killings; fit with eyewitness description; his own "alibi" coinciding with key parts of state's narrative that he driving near the scene at the time etc etc

It is not that belief in innocence is unreasonable - it is more often that no reasonable or even coherent explanation for any of the evidence is offered without getting into bizarro conspiracies or ignoring coincidences the staggering statistical improbability of which make the chance of two identical snowflakes being found on a knife sheath seem common-place.