On the season finale of lost why did Keamy blow up the boat ?

Both answers are half right....

The truth is...that it was all part of the Plan B that they were tlaking about a few episodes before....When keamy came back from the island the first time he went and got a folder from the control room with a Dharma emblem on it....and said its time to go to Plan B....also keamy knew that Sayid and Desmond were on the ship and by having a detonator rigged he could bargain his way out of a situation....but he didn't count on Ben not giving two sh**s....still I think the rigged C-4 was a way for Keamy to "leave" no witnesses....after all if he is dead, then he feels the mission was failed, because its already shown that he doesn't think anyone else on the ship is capable of following through with whatever it is that Widmore ordered them to do.

If you remember, Keamy tells Ben that he has a detonator attached to a heart monitor and if he dies, the freighter will blow up. Ben stabs him and Locke tries to save him, but he dies. Keamy had the attitude that if he dies, he's taking people down with him.

exactly what angelo said but I think keamy used it as a ace in the hole, a bargaining chip. so if he get caught he could play the "keep me alive or blow up the ship card". but it didn't work for him because ben was playing the "killed my daughter therefore you must die and all else be damned card." which trumps keamys card. :)

Great question...