I wouldn't buy into every gun owner horror story you hear anecdotally on the internet. Much of that is urban myth, or the exception rather than the rule.
Your instructor should have covered deadly force statues in your basic safety class, and you should be very familiar with the scope and limitations of your right to defend yourself. If not, I highly recommend you attend a class that covers this, whether it's GOAL's Art of Conceal Carry or one of the MA firearms law lectures that are offered (LenS, Attorney Cohen, etc).
However, I also recommend that EVERY gun owner establish a relationship with a attorney who specializes in firearms and self defense, especially if you plan to conceal carry or keep a gun ready for self defense. Right, wrong or otherwise, you don't want to be thumbing through the yellow pages from a holding cell downtown. Establish a relationship, put a retainer down if necessary, and develop a response plan so that you know exactly what to do if you're forced to use your firearm. That gun on your hip on in your nightstand is a HUGE potential liability, and you need to know when and how you plan to use it, when you don't use it, and what you need to say and do (and NOT say and do) when/if you need to use it.
Too many people treat buying their first firearm like any other new toy, and they spend tons of time researching the gear and accessories but ignore the wider responsibilities and ramifications of gun ownership. Don't make that mistake.