Is there such a thing as an ethical reason for unauthorized access to a computer system?

Meaning of key word here is "unauthorized": We don't have access to use the system. But you are try to accessing an Unauthorized system which is illegal, can be ethical. I think, although not legal in point of where you have to decide of right and wrong. Like if you want to getting information from criminals personal file to catch him then i would be right.

Yes if an administrator want to fix a problem something going on in his/her network wrong,then he/she can access the systems which is attached to network. Like if somebody is trying to access your's network or wifi getting password then you can stop to them use ethically,stop her/him system access to attaching your's network.