Not really an exception to the rule, Nancy.
The ruleset must be set up to ACCOMMODATE individual variations in order to be optimally moral. Like our self-defence laws. We are allowed under the law to defend ourselves, and if it's necessary to kill someone who is attacking you with lethal force, it is legally justifiable to do so. This is not an "exception to the anti murder law" so much as it is a recognised special circumstance that warrants its own rule.
[FW] Spider AL
Hewwo, meesa Jar-Jar Binks. Yeah. Excusing me, but me needs to go bust meesa head in with dissa claw-hammer, because yousa have stripped away meesa will to living.