I don't think the introduction leaves much for interpretation, it very much makes out as though he is the killer. It looks like his life has turned to shit by looking at him, I am guessing from here on out it is just downhill, with that shoulder blow messing up his pitching. I expect she breaks up with him and that is the final straw in his crumbling life.
You don't stab someone multiple (Guessing multiple even though it isn't stated, but look at her...) times in the street for a mercy killing and I don't see why he would be framed when he already looks at rock bottom.