Is snooping considered a sin?
Everyone has secrets (or, at least things s/he would not post on a Facebook wall for everyone to see). Unjustly taking these secrets is a form of theft. It is not the same as theft of gold, where the thief denies the owner access to his property, but it is still a form of theft. People who “snoop” have taken something (information) that does not belong to them, and which they have no right to take.
If it’s done just out of nosey-ness and not to try and help someone, would this be a sin?
Did you mean to word this the other way around? If so, this sounds like an “ends justifies the means” argument, which is not well justified in Catholic theology (but it’s not categorically condemned, either). This is really a matter of circumstance. Few people would argue that a parent has the right to search a minor child’s room to look for drugs if the parent has reasonable suspicion that the kid is taking. But if this same child becomes an adult and establishes his own residence, the parents would have NO right to barge into his home and search for drugs, even if they suspect he is taking.
But to snoop on someone just to see what you can discover is
certainly an offense against this person and is **certainly **sinful.
What about if you read someone’s email? Is this a sin, and if so it mortal or venial?
For a sin to be “mortal” it must meet three criteria: It must be a serious matter, such as a violation of one of the Ten Commandments (and theft is such a violation), and it must be committed with full knowledge of the seriousness of the offense, and it must be committed with full, deliberate, and complete consent of the will.
Theft of knowledge could fit the first criteria. The determination of culpability for mortal sin depends on the snooper’s disposition regarding the other two criteria.