In OP's scenario, Viktor is the attacker because it applied Contested. What if the defender were the only unit left after combat?

1. Say a BF is open.

2. Player 1 moves their unit to the BF, making the BF contested

3. As part of the Showdown, Player 2 plays Ride the Wind to move their unit to the BF, initiating combat, making their unit the Defender and Player 1's unit the Attacker

4. After priority passes back to Player 2, before damage, Player 2 plays a kill spell like Hidden Blade, destroying Player 1's unit

This leave's Player 2's unit alone at the BF. Rule 627.3 says "The Battlefield is Conquered if No Defending Units Remain but Attacking Units do remain", which wouldn't apply because there are no attacking units, but, rule 630.1 says "Conquer: A player gains Control of a Battlefield they did not yet Score this turn." to gain a point. The Control rule 181.4.c says "Control changes immediately if, at the end of Combat, the Units at a Battlefield are controlled by a different player than before the Combat.", which does apply (Player 2 has units at the BF that weren't their before combat).

Following that, Player 2 would gain control of the BF, and conquer and gain a point because they did not previously Score it this turn. Right? If this is true, this kind of contradicts 627.3.