Does trample damage carry over to the player if all blockers are killed by first strike?

When a creature with both trample and first strike (or double strike) attacks and is blocked, combat damage is dealt in two steps: the first strike damage step and the regular combat damage step. During the first strike damage step, the attacking creature deals its damage, potentially destroying all blockers.

Under CR 509.2, a trample creature must assign lethal damage to each blocker before assigning excess damage to the defending player. However, if all blockers are already dead (removed from combat) by the time the regular combat damage step occurs, the trample creature is effectively unblocked for that step. CR 702.19b specifies that trample excess damage goes to the player or planeswalker being attacked.

Because the blockers are no longer legal damage recipients in the regular combat damage step (they are in the graveyard), all of the trample creature's remaining damage — if it has double strike — goes to the defending player. If the creature has only first strike and not double strike, it already dealt all its damage in the first step, so there is no second damage assignment.

Concrete example: You attack with a 4/4 creature with double strike and trample. It is blocked by a 2/2. In the first strike damage step, the 4/4 assigns 2 damage to the 2/2 (lethal) and 2 damage to the defending player (trample excess). The 2/2 dies. In the regular combat damage step, the blocker is gone, so all 4 damage goes directly to the defending player. Total: 6 damage to the player.

If your creature has only trample and first strike (but not double strike), it deals all its damage once in the first strike step. With trample, it still assigns lethal to the blocker and excess to the player in that single step — the blocker dying afterward is irrelevant since damage is already dealt.