Full Idea
A justifying reason will show that what you did, all things considered, was the right thing to do; an excusing reason will not justify, but will give some excuse to explain why you did what you did.
Gist of Idea
Justifying reasons say you were right; excusing reasons say your act was explicable
Source
Peter Goldie (The Emotions [2000], 6 Intro)
Book Reference
Goldie,Peter: 'The Emotions' [OUP 2002], p.142
A Reaction
There are also internal reasons before the event, and explicit reasons afterwards. A mistaken justification might still be an excuse.