How violent is Snape, really?
Maybe Severus couldn't kill. The only way around it was acting the coward - and then to turn double agent. How could he possibly be part of killing missions when Dumbledore thought he was his agent? That way he could nicely stay behind, being even commanded to do so by Voldemort.
This may have given him the reputation of letting others do the dirty work for him, like Bellatrix implied at Spinner's End. It might hav been the cause for his turning, or a part of it. He would not have been the first fanatical youth who got cold feet after having been lured into a group that promises power and greatness - and later demands violence and murder.
Otherwise, how can he be a ruthless murderer and at the same time so honestly upset about Sirius wanting to kill him. 'Capable of murder at 16' he reminds Dumbledore, with a righteous outrage that I believe was genuine.
So how could he perform the most unforgivable of the curses?
Aurors can perform the unforgivable curses. How do they learn, with animals?
Would Dumbledore really have trusted a murderer?
But how did Snape get 'so close' to Voldemort? How long has he been that close? Since the killings? Were some of these killings set up? To make Voldemort trust him more, to see him as advisor?
How about Sirius? Did Snape know about V's plan to lure Harry into the DoM? 'He was ordered to stay behind...' Was that a general order for him, or specific for that 'mission'. How fast exactly did he call in the order? Why did he call them at all? He could have found excuses. No one else would have understood Harry's cryptic remarks. The kids would all have been killed if the Order and DD hadn't arrived.
Where is book 7? Whine!