Hi there! Apologies for taking such a long time to deliver your requested review!
I'll start out by saying that I loved the repetition you made use of, particularly in the first half of the one-shot. It really added to the mood of the story--it made me feel a sort of futility or inevitability that I think applies nicely to Snape's story. I would have thought that so much repetition would get tedious after a while, but it didn't at all!
I guess that speaks to your first area of concern, which was flow. Overall, I thought it flowed nicely and had a great rhythm to it. The only suggestions I have are that a) in parts, I felt like you dwelled on certain subjects for longer than was necessary to hit the message home, and b) it was occasionally difficult to tell who was the focus of the narration. This was mainly in the section between the paragraphs beginning with "But then he'd look at him with those eyes..." and "Then he'd lift his head and glare at him..." It's a very powerful image, but I thought it might strengthen the middle section of the story if you were to shorten it a bit, because talking about looking at Harry eventually started to feel a bit redundant. Also, I was unsure in certain parts of this section exactly who was doing the looking. I wasn't sure if it was supposed to be all Snape looking at Harry, or Harry looking back at him, or a combination of both. I think you just have to be careful with all the "he"s, because if it's not totally clear who's being referred to, it can get confusing.
As for grammar, this was really well-written. I hardly noticed any errors, but the main one that stood out to me--and actually, I don't even know if it should be called an error so much as a slightly ineffectual choice of words--was in the sentence, "He looked so much like her husband and he could hate him for a bit." I think what you're trying to get at here is that, because Harry looks so much like James, it allows Snape to hate him, until he is reminded of the presence of Lily on Harry's face. I think you could maybe revise the "hate him for a bit" portion of the sentence to get that across more clearly.
I think characterization was your main concern with the story, so I'll try to give you some feedback on that, too. First off--Snape is one of those nightmarishly difficult characters to write in line with canon. I remember I wrote a chapter in his POV at one point, and I had a hard time getting to a point where I felt like I had captured his character properly. I think you captured the bitterness and regret he felt really well. You covered all the different aspects of his relationship with Lily, which is quite a lot to capture in a single one-shot, and I was really impressed with that! The only comments I can offer on characterization are borne out of my own interpretation of Snape's character, so you can take them or leave them. :P
The one thing that I felt was a bit off in his characterization was that Snape didn't quite seem bitter enough. Again, this is just my view of his character, but I always think that he would have had this constant sense of injustice about losing Lily, rather than giving in to the feeling that "she deserved anyone but him". I think he would have forever blamed James for stealing her away (which you did emphasize in parts), and even blamed Lily herself for not giving him another chance. I think Snape seemed to be very good at putting the blame on other people, even after Lily died. I do agree with the basic premise of your story, which is that he regretted giving Voldemort the information that got Lily killed so deeply that he was never the same afterwards. I just think that there's a line between regretting something and being truly apologetic about it, and maybe everyone's answers about what side of that line Snape falls on are different! :)
Overall, though, I do think your characterization of Snape was quite good, and so was the one-shot more generally. Great work!
Author's Response: Hello! It's okay, I understand that you probably had a lot on your plate!
I thought the use of repetition would get tedious as well, but I'm glad to hear that it didn't! :D
Hmm... Okay, I understand what you're trying to say. When I have more time I'll go fix it up, a few people have said this so I think I'm going to have to change it. Thanks for the tips! :)
Okay, I'll also change that around! You're very helpful! :D
Snape was a nightmare to right, he's so complex! Okay, so not bitter enough - got it! I wanted to put a new spin on Snape but I still wanted him to recognised as canon-Snape. So thank you again! :D
Thank you so much for this extra-long review, I'm sorry my response is so short! Your tips really helped, thank you! And thanks for taking the time to review this for me. :D