Snape would reason correctly that if Voldemort wins, he would punish Snape anyway, for his betrayal during Voldemort’s absence. So Snape would try to help Dumbledore and not Voldemort as long as he has a hope of success.
Alternatively, Snape would reason correctly that Dumbledore and the Order of the Phoenix failed to beat Voldemort before, having to be rescued by an unrepeatable deus ex machina, and they’ll fail to beat him again, especially now that he’s returned with greater knowledge and power, and proven immortality.
Alternatively, Snape would reason correctly that Dumbledore and the Order of the Phoenix failed to beat Voldemort before, having to be rescued by an unrepeatable deus ex machina, and they’ll fail to beat him again, especially now that he’s returned with greater knowledge and power, and proven immortality.