Primeval slayer is a mechanic put in place to counter exactly what you're complaining about. If you thought the enemy team getting 1 free stack when you had 5 stacks was bad, imagine if you did the same amount of damage to the Primeval no matter what. Then the team that summons their Primeval first will truly be at a disadvantage.

I’m genuinely stunned as to how these god forsaken forums managed to complain about a mechanic that gives you EXTRA damage. Jesus.

I mean, there are a LOT of things about Gambit which need to change. Win Round 1? You do 50% less damage (and take more incoming damage) in Round 2. Pull your Primeval first? Guess what, that "Slayer" buff has a counteracting damage debuff (might only be Round 2, didn't stick around long enough to thoroughly test every situation), along with giving the other team free Invasions (and a lot of us can't take increased damage from Envoys, get battered by a Primeval, fight a bunch of ads, AND deal with constant Sleeper-wielding overshielded scrubs popping up all over the place while trying to kill the Nightfall-esque boss). Ever notice how sometimes the other team is doing poorly, then suddenly skyrockets in Motes? That's because the game spawns special glowing "catch-up" enemies if they fall behind, and delays enemies spawning on your side. Ever feel like the game is actively preventing you from winning? That's because it is, at every step. It's hard-coded to switch advantages around so often I'm surprised every game doesn't end in a tie - that'd be a great metaphor for Gambit in general. The only way to win is not to play.

Doesn’t primeval slayer REWARD the team who summons their primeval first? The team who gets the primeval second gets a fraction of the leading teams

Get. Gud.