Conviction variance

Sometimes the justice system actually can't know whether somebody's guilty or not, but because a serious case has been brought up, they have to decide something.
I think there are variances of convictions:
Guilty,
Not guilty,
Safe boundaries.

When a murder has happened, there's an instance when it's not safe to let someone loose into the society even though he/she might not be the murderer. There's substantial prove even though not at 100% mathematical/quantum certainty that he / she was the murderer, to not let this guy loose.
I think in this instance the law should be able to suspend him/her from common societies for the sake of safety. But I think it's wrong for the law to convict this person guilty.
For example there were lots of witnesses saying that this guy was at a place during the night. But there were also some who said that the person was at another place.
Well it was night and some people looks the same when he/she was drunk and/or using similar make ups.
So even though there were no lawyers to present the case, judges who are rational could consider this possibility and should be undeterminant because it's just right to do so.