Sometimes prudent tactics make it advisable to try and retreat a distance before confronting a continuing threat, but this only muddies things at a time when clarity of thought is crucial.
Preserve your OWN life first, worry about your attackers life not at all. If they initiate force against you, they forfeit their Right to keep breathing.
Bears repeating. Retreat is a good tactical option sometimes and sometimes retreating until you are forced to use lethal force is suicidally stupid. In some situations retreat can actually escalate violence. Legislators who mandate it as a one-size-fits-all solution aren't living in the real world.