Word or phrase for the moment/reason/motive you reveal to someone why you had acted a certain way prior to parting ways
[the] reveal
the moment in which previously withheld information about characters or plot is unveiled.
I have a couple suggestions for the reason:
Vindicate:
to show that (someone or something that has been criticized or doubted) is correct, true, or reasonable
http://www.merriam-webster.com/dictionary/vindicate
Justify:
- to provide or be a good reason for (something)
- to prove or show (something) to be just, right, or reasonable
- to provide a good reason for the actions of (someone)
- http://www.merriam-webster.com/dictionary/justify
I think the villainous info dump is usually referred to as monologuing and the link that @MετάEd shared from tvtropes is pretty much spot on.
I would go with parting shot:
A final remark, usually cutting or derogatory, made just before departing.
The parting shot can (and usually does, at least in movies) include a revelation.