[spunkets]

"Seems like axions have enough mass to form the 2 particles and some energy is usually given off during decay. Unless the decomposition is endothermic but the decay apperars to go from high enery to lower energy particles."

The mass of the axion is between 10^-3 and 10^-6 eV. That's very small. All the energy is contained essentially in the momentum of the particle.

E = p*c.

Thermodynamics isn't important here. The energy of the particle before the decay equals the E after the decay. The momentum of the tiny mass is turned into a particle pair. It's the reletivistic mass which is is important here. The speed is very close to the speed of light so the mass is 10⁶ to 10⁹ times higher.

[spunkets]

The energy of the pair is over 1.02 MeV.