The evidence is subjective. Let's say Fred is suspiciously missing, Ted has the motive to kill Fred and has been quoted as wanting to kill Fred. That is still evidence that Ted could've killed Fred. It's subjective evidence but it is still evidence. Now it is a matter of believability. can the evidence be believed? There are cases were people have been convicted of murder with the absence of hard evidence like a body or murder weapon.
Sure folks get convicted on circumstances. An atheist makes a choice not to convict based on circumstances. Lots of possible reasons for Fred's absence. Having a motive is not proof of acting on it. Lot of people get wrongly convicted. Circumstance is not IMO a good basis for conviction. Obviously other folks think differently about it.
I've been wrong enough making decisions based on claims and circumstances that I no longer view these things as a reliable basis for decisions.