Germany has two kinds of punishment systems.
The usual system is ordinary punishment system, which has life sentence (for murder and some other crimes), up to 15 years of prison, and fines (based on the income, up to 180 days of income are possible).
However, for more than 100 years we have introduced a special system for young offenders. You always get punished according to the ordinary system when you are above 21 years old. Between 18 and 21 years, it depends on your personal development, if you still seem to act as a person that isn't grown up, you get punished according to the juvenile system.
The juvenile punishment system is appliccable for everybody between 14 and 18, and, as said, can be appliccable to people that are up to 21 years old. It has another purpose.
The general punishment principle of the German state is that of "general prevention", which means the purpose of punishing people is to prevent other people to do the same offenses. Re-integration of offenders is only a secondary requirement.
The juvenile punishment system wants something else. It wants to bring young offenders back on the track of legality, therefore, punishments are only used if no other alternative is seen. Instead, for smaller offenses, young offenders must work in social organisations like hospitals, help old people and something like that, or they must confront their victim and try to apologise for their behaviour and do everything to bring the situation back to normal.
The reason is, if young people are sent to jail, they meet real criminals, starting a criminal career when they are out again. The law thinks that chances are good that people who do smaller offenses when they are young have good chances of never doing something illegal again. Everybody has done something stupid when he is younger, and the position is that a flexible system can work out better than the strict criminal system.
However, working at a hospital for 2000hours is no proper punishment for severe crimes, therefore, the law also has provisions for youth jails, up to 10 years are possible.
So, 10 years are the maximum possible for people under 18 who commit murder. In this case, the courts came up with a judgement which is very close to the maximum punishment, this seems not that much out of proportion.
Bear in mind that this guy will be sent home to Turkey directly when he is released from jail, so he has more than enough to worry about.
It can, of course, be discussed whether the juvenile punishment system in Germany has achieved the intended results. Very often, people who are getting criminal when they are 14 remain criminals, and the system could not change that very much. But I think the idea of the system is good. I am thankful that we do not have 16 year old guys to sit on the electric chair.