No, the real problem is that because you consider vehicular manslaughter sentences too low, it should legitimate the murder of the drunk driver or at least make it fair. I agree that 2 years is way too low for what should be a crime (if it's not already in US books) and for which sentence should be way harder and close (I don't know the US legal terms) manslaughter/murder ones. But how does it legitimate his murder ? Because that sentence is objectively too low, killing the man is only right/fair ? So should the parents/family of the drunk driver go at the father for killing their son who should have been lawfully convicted by a court for his crime and would have still been alive ? Where does revenge stop ?
Yes, killing the man is right and fair in my opinion, regardless of the sentencing

The deaths of those two children (11 and 12 years old) could have been avoided, but his complete lack of respect for the well being of others is why he's dead and I have no problem with how the father handled it. I brought up the penalties of DUI to highlight the fact the system is broken more than anything else and justice would not have been served, justice is a life sentence without possibility of parole, and that wouldn't be on the table. So, the alternative is him being shot and killed at the scene.
If his parents want to take revenge, tell em like the Bride told Vernita Greens daughter -
It was not my intention to do this in front of you. For that I'm sorry. But you can take my word for it, your mother had it comin'. When you grow up, if you still feel raw about it, I'll be waiting.
Murder (not talking about self-defense obviously) should never be legal (and for such I am anti-death penalty too but that's another debate) no matter what are the circumstances. He got away free but should have been convicted and the sentence should have been light because of the circumstances. It's all in the symbol : him getting free is telling that the citizens have the right to kill even if it's just "revenge". Him being convicted + lenient sentence would have tell another story : murder is not tolerated but we can understand why he did that.
There are times when murder is acceptable beyond self defense, you molest someones child and you're caught by the parent, your brains get splattered across the wall then it's

status to me. Nah, this man going free is more or less a case of

to all the drunk drivers out there. The punishment for DUI & killing someone is not severe which is why we have so many instances of it in this country. This man should not be convicted, a symbolic conviction+ light sentence just makes a mockery out of the system you're talking about.
PS : I never stated nor assumed that my country was more civilized than yours
You do live in a more civilized country, I wasn't being sarcastic

If you kill someone where I'm from then you better be ready for that get back, just how it works.