bofh
Banned
You are misreading the plea agreement. He is pleading guilty to excessive force, not Second Degree Murder and Obstruction of Justice. Read paragraph 2.a:
That, in North Charleston, South Carolina, the defendant deprived Walter Lamar Scott, Jr., of the right to be free from the unreasonable and excessive use of force by a law enforcement officer, a right secured by the Fourth Amendment to the United States Constitution;
The mention of Second Degree Murder and Obstruction of Justice is only the sentencing recommendations the government is going to make to the judge because that was basically the final result of Slager's actions. Settling on excessive use of force removes any minimum sentence requirements.
Yes, you are right. When I was writing the post, I was reading the sentencing guidelines at the same time in trying to understand the 3-level reduction.