If a defendant can prove a successful justification for his actions, he will be fully cleared from criminal liability.
In case of excuse, the act is considered criminal unlike justification but the accused person shall be spared from the consequences of crime. Example, Intoxication, insanity etc.
Mitigating factors has been used to lessen the punishment only. Suppose in case of murder mitigating factors could potentially lessen the punishment of death sentence to life imprisonment.