Why did Cas go to stab Lucifer anyway? He knew it wouldn't kill him.. Did he just want a little revenge first?
Literally the only real reason I can think of (ie. that isn’t explainable in that the plot could have been written in many another way to get to the ending with Lucifer and Mary in the AU and Jack born) is that it was done this way just so he’d get stabbed ie so we got our break up moment to further the Destiel plot and so that we would get a Dean / Sam Cas / Jess parallel with Sam dragging Dean away…
Ie. there was no real reason for it to happen except to make it romantic.