well mechanically, its a mechanic's responsibility to tell you whether a car is "good" or not, well that's why you went to that mechanic anyhow! (to get the car checked!)
however, it is also YOUR responsibility to check the reports such as carfax and other online reports that show whether the car has been in previous crashes or not
you should also have done a lien check, gotten a carfax report, etc
thus the mechanic is not guilty for not telling you, he probably didn't know himself (which means he's not a good mechanic, you should switch!)