A lot of readers think Snape did what he did only out of his love for Lily. The books, however, suggest that Snape didn't really care just about protecting Lily’s son, through the extent to which he was dedicated to Dumbledore’s mission -- beyond protecting Harry; or, probably, to the point of being ready to fight Voldemort.
Based on what the books say, I think we missed a ‘Voldemort v/s Snape’ duel because of a momentary lapse of judgement by Snape.
Dumbledore: "If there comes a time when Lord Voldemort stops sending that snake forward to do his bidding, but keeps it safe beside him, under magical protection, then, I think it will be safe to tell Harry [that he’s a Horcrux] ."
Voldemort: "… do you know why I have called you back from the battle?"
"…Harry saw Snape’s profile: his eyes were fixed upon the coiling snake in its enchanted cage."
Snape: "No, my Lord, but I beg you will let me return. Let me find Potter."
"…it is of you that I wished to speak, Severus…"
"But -- let me go and find the boy, my Lord."
"Snape was not looking at Voldemort now. His dark eyes were still fixed upon the coiling serpent in its protective sphere."
"My lord -- let me go to the boy --"
"While you live, Severus, the Elder Wand cannot be truly mine."
"’ My Lord!’ protested Snape, raising his wand."
"…Voldemort swiped the air with the Elder Wand… It did nothing to Snape, who for a split second seemed to think he had been reprieved…"
"Dumbledore’s man. Through and through."