Well I can see you are all looking at it on face value, so I will go with another route ...
Sarcasm has many uses in life but one of the many ways is that of concealment of feelings ...
While I can see the face value of this as well, I can also see a subtle undertone ...
On face value
Snape despises
Harry because he is so like his father, the same father that tormented
Snape while at school ... while
Harry appears benign in his action whereas his Dad was more malicious,
Snape can't see past this and makes
Harry's life a misery to salve the injured pride and sores of the past damage inflicted by James ...
Looking at it deeper and as always looking at the controversial, isn't it possible that
Snape is using this sarcasm to conceal his uneasiness' that
Harry keeps placing himself in danger?
If, and I say this is a big if, If
Snape is looking out for
Harry - he may feel very angry and worried that
Harry went wondering off and could have fallen pray to Black or indeed the Death Eater movement ... his comments look to put
Harry down but it is said in a fashion that makes
Snape be sarcastic but underneath he is relieved
Harry is OK ...
As I said, I don't expect people to agree here and also expect the usual closed mind answers but the world is a lot smaller to the closed mind ...