It's da Bomb
Charles Widmore dug up a bunch of bodies, stuck them on a plane and submerged them all in the deep blue sea so that no one would ever know what really happened to Oceanic Flight 815. Widmore has been trying to find the island for many years. Michael was told by the Others that if he does, everyone there will die. If Michael wants to redeem himself for betraying his friends, then he cannot allow Widmore to succeed. That explains the big bomb he's stashing aboard the freighter.