With regard to the struggle.
In context of what we have on the video, it is quite clear contact was made between Scott and Officer Slager. Who first made the contact, is arguably moot. The point is, there was a struggle. And the officer appeared to be over-powered during at least part of that struggle. Moreover, he was unable to physically detain Scott. And once that occurred, all bets were off. He was unsurprisingly pumped, that is, his adrenaline had kicked in when contact was made. At which point, he is in the fight mode, as opposed to flight mode. That is, his behavior is arguably more visceral than cognitive. His stance, when he is taking aim, seems clearly singular. Focused.
Does this make the shooting right? Of course not. Understandable? I would pose that, to some degree, it does.
Eta ~ adding video of struggle for clarity:
https://www.youtube.com/watch?v=cx2dZkrwqHU