Sooner or Later =  Eventually,  certainly although one cannot be sure when, at some time in future.  To explain further, you don’t  know exactly when something will happen but you are certain that it will happen.

• If you do such a mean thing, you will be punished sooner or later.
• The culprit will be brought to book sooner or later.
• You will have to tell the truth sooner or later.
• The accused has escaped from the police custody.  But he will be caught by the police sooner or later.