My friend, please don't blame yourself for his death... Trust me. It wasn't your fault.
Without going into gas diffusion across membranes, partial pressures of dissolved gases and also the fact that these stagnant and muddy streams are Bettas' natural habitats, I can assure you that you DID NOT gas your fish with CO2. The only way to really do that is to inject pressurized CO2 into a system. Even completely stagnant water (see the photos, where Bettas have been collected), can sustain labyrinth fish quite comfortably.
If your CO2 was too high, your shrimp would have died way earlier. Also as CO2 would turn into carbonic acid when dissolved in water, your pH would drop and you would have seen that in your testing. There was another reason for this tragedy, and it was not something of your doing...
Not being a smart@$$, I just don't want you to beat yourself up over his death...
May your fish rest in peace, brother.