A few other things to consider:
On Voldemort agreed to spare Lily, I've always thought that was a gift or something for delivering the Prophecy, and Voldemort did mention that he rewarded those who were loyal to him.
Voldemort meant for him to be a spy and take up the DADA post, logically that meant he would have had to have a relatively clean slate. From the numbers JKR has given us (though she can't do math for shit), it seems that there would have been at least over 100 Death Eaters. At Karkaroff's trial, when he named the Death Eaters he also specified what they were known for (torturing mostly), but he said that Rookwood was a spy and he didn't say anything about what Snape did.
In GoF, Sirius was surprised to find out that Snape was a Death Eater (and said that as far as he knew, he wasn't even accused). You'd think that if Snape used Sectumsempra on the battlefield often, the Marauders would have been able to recognize it (seeing as how it was Snape's "specialty").
JKR has said of Snape (as a response to "Can he see thestrals?"): "He can see thestrals, but in my imagination most of the older people at Hogwarts would be able to see them because, obviously, as you go through life you do lose people and understand what death is. But you must not forget that Snape was a Death Eater. He will have seen things-" -Edinburgh Book Festival.
In both JKR's quote and Dumbledore's, JKR has specified that he's watched or seen people die, both times in which she had the opportunity to say that he killed people. In response to the thestrals question in particular she could have easily said something along the lines of "Well duh, he's killed people as a Death Eater". Make of that what you will.
All of the above is somewhat sparse evidence, but JKR usually chooses her words carefully and has had multiple opportunities to state whether Snape had killed, yet she hasn't.
And finally, Snape didn't kill Sirius even when he thought Sirius was responsible for betraying Lily, and that he was a danger to the Trio (remember, Sirius had broken into the castle multiple times, stood over a student’s bed with a knife, and Snape walked in on Ron with a mangled leg). Later, when he regained consciousness, and he thought that Sirius had attacked him and tried to run off with Harry, he still didn’t kill him. This is a stark contrast to how Sirius and Lupin were about to kill Peter in cold blood in front of three kids, even though Peter was the only evidence that could clear Sirius. That sort of makes them more willing killers than Snape. Yeah, Snape wanted to hand Sirius over to the Dementors when he thought he was guilty, but that was what Harry wanted to do to Peter, and rather than calling the Dementors over he took Sirius back to Hogwarts to have some sort of legality behind it.