So if the potential hazard is almost certain to end in a fatality the risk rating would be a 25
If you apply a control measure, you will reduce the likelihood (frequency) of an incident, the control should also reduce the consequence should an incident still occur after applying the control.
If your job step was to disconnect the electrical power feed into a machine, and you applied no control measure what soever, you would be almost certain to electrocute yourself and end up with a fatality. Therefore a risk rating of 25.
If before undertaking this task you applied some control measures like disconnecting and isolating the power, the likelihood of electrocution would be reduced from almost certain to rare. So if you did have an incident disconnecting the power after you diconect4ed and isolated the power it would probably only be a minor incident (Possible a scratch) thus reducing the risk rating from a 25 to a 1.
If by applying control measures you can not reduce the risk rating to fall into the Low (Green) area, then further consideration is definitely required.