You're right when you say that one person's good deed in one domain don't make him immune to criminal charges for unrelated matters. However, Assange did not just did a 'good deed', he became a very high value political target and most of the time such people are protected from unrelated charges, since it is very easy to false flag them/burden them with bs to keep them from doing their job. For example in France the president has partial immunity. The case of diplomatic immunity also comes to mind.