Well, yes, I can see this, too. And, in any case, it's quite clear that Severus does not trust Voldemort to spare Lily. If he had, he would not have gone to Dumbledore to beg for her life, would he?

Reading between the lines, one could postulate that young Severus was already thoroughly disenchanted with the Death Eaters by the time he was 20. But he also knew what his reward would be if he tried to leave.