It depends on HP before the hit.
Essentially, an attack can be broken down into a couple phases. First, you select a target. Then you make an attack roll, and determine whether the attack hits or misses (or crits). Then you resolve damage and any other effects (mostly these only happen on a hit).
The damage effectively happens all at once (unless a particular effect explicitly says its damage happens later), so when you're checking whether to add the extra damage for Colossus Slayer, the damage from your hit hasn't been applied yet; Colossus Slayer damage happens at the same time as the rest of the damage for the hit.
The other way of thinking about this is that if Colossus Slayer depended on HP after the hit, it would pretty much always deal its extra damage, unless your hit managed to do zero damage. At which point, they might as well change the feature to read, "You deal an extra 1d8 damage when you hit with an attack." Since the feature isn't phrased that way, it seems unlikely that that's how it's intended to work.