My guess is that when the criminal case is adjudicated Slager will take a plea to manslaughter and be sentenced to a term in prison for a number of years.
If Slager had a history of complaints, then his superiors have some responsibility for allowing him to continue without some close supervision at least.
I’ve seen a couple of conflicting mentions about the Coast Guard service. An article on CNN states that the Scott’s brother said he served in the Coast Guard (but then it doesn’t an a direct quote from the brother stating that.) http://www.cnn.com/2015/04/08/us/south-carolina-who-was-walter-scott/
The original article that appeared in the local paper about the shooting stated that the officer, Slager, was former Coast Guard but didn’t mention any service history for Scott. http://www.postandcourier.com/article/20150406/PC16/150409558/1177/north-charleston-police-say-officer-who-fatally-shot-man-pulled-him-over-because-of-brake-light
Either way, the tone of that original article certainly places a different spin on the officer’s actions prior to release of the video.