Not only must self defence be reasonable and proportionate to the threat reasonably perceived, but the allowable extent changes as the circumstances change.
For example: If someone attacks you with a knife, you may be justified in the use of deadly force for self defence. Once that knife is dealt with (say you disarmed them), that justification will most likely be gone.
For example: If someone attacks you with a knife, you may be justified in the use of deadly force for self defence. Once that knife is dealt with (say you disarmed them), that justification will most likely be gone.