Method #1
Kill Sebastian by placing a C4 on the hearse and detonating it.

Method #2
Kill Sebastian by killing him in the bathroom or sniping him from afar while he's in there.

Method #3
Kill Sebastian by pushing the explosive barrel down the chimney.

Method #4
Kill Sebastian by shooting him through the skylight above him.

Method #5
Kill Sebastian by shooting him or sniping him into Pierce Hermans Grave.
(A Gardener will arrive shortly to bury the grave)