I try to be very tolerant of the unprompted self-justifications and maybe just ask a couple questions about it. At some level they feel a change is warranted, and humans change their minds messily over years, not instantly during arguments.
Yeah, I mean they are not fooling anyone, if they bring the topic up on their own trying to tell me why they eat meat it is quite obvious they have a guilty conscience and are trying to justify it to themselves more than me.
Your approach is a lot more conciliatory though, I am usually so annoyed that I just question why they are so defensive they are telling me those things unprompted.