Malbim: (a) He was among those who rebelled against David, for which one is Chayav Misah. Surely you will pardon me among the entire nation, who rebelled! (b) His own sin of cursing the king. This was severe for three reasons. (a) The sin itself - "Asher He'evah." (b) The time (the day you learns from Yerushalayim), tired and weary. (c) The intent. One who rebels intends to cast off the yoke. Cursing and stoning is only to pain the king.