This post is the (much shorter) second of two, and is simply meant to demonstrate the similarities between Ahithophel and Judas, and their respective betrayals. I was expecting to find more than this, but didn't.
First, the obvious: each betrayed God's anointed king to those who desired his death, and each committed suicide (by hanging) rather than repent and seek mercy. However, each was ultimately foiled. So far as Absalom and the Pharisees: each hated the true king out of jealousy and sought to take his place, and each had grievances against the true king (whether real or imagined).
Well, that's all for now; on to the next question, which I'll invent sometime soon.