The Constitution specifies what is lawful. If the government's actions are contrary to what the plain text of the Constitution requires, then the government's actions are not lawful. Courts have the authority to "fill in the gaps" in some places where there exist bona fide ambiguities in the law (though in criminal cases, due process requirements would generally require that ambiguities be interpreted toward the defendant) but they have no legitimate authority to insert their own judgment in places where a superior law would demand otherwise.
Suppose there is a jellybean-counting contest, and Al, Bob, Chuck, Dan, and Earl submit guesses of 67, 746, 747, 748, and 978, respectively. The jar contains 500 whole red jellybeans and half of a red jellybean, along with 246 whole blue jellybeans and half of a blue jellybean. Joe is the official jellybean-counter. If neither official rules nor previous practice dictate how partial jellybeans should be tallied, Joe would be within his authority to not count them, count them as whole jellybeans, or count them fractionally. Depending upon which policy Joe adopts, there could be 746, 747, and 748 jellybeans; either Bob, Chuck, or Dan could legitimately be declared the winner; absent evidence of bribery or other bias, everyone should accept the decision without complaint. If Joe were to decide that there were 48 or 987 jellybeans, however, absent something in the official rules, that would not make Al or Earl legitimate winners.
The question is one of authority. Where does the Constitution give you the authority to declare a law to be "unlawful"?
Allow me to illustrate: In professional sports events, such as basketball, there are specific officials who have been given authority to declare a move illegal for one reason or another. A referee may declare a foul, double-dribble, traveling, etc.
The actual RULES are written and everyone knows them. There's no question about those.
But who has the AUTHORITY to call fouls? Who has the whistle? Who wears the black and white stripes?
By contrast, would it be OK for one of the players to bring a whistle and call fouls (or other violations) while playing? No. And it's not because a rule is right or wrong, or a question of whether or not a violation occurred. Neither of those issues matters, because a player simply doesn't have that authority.
It's my contention that a juror does not--and SHOULD not--have the authority to interpret the law and declare one to be "unlawful" or un-Constitutional. That authority is EXPLICITLY and SPECIFICALLY granted to the courts, up to the Supreme Court.
A juror is not granted that authority when sworn in as an active juror. He is not given that authority by the Constitution. If we are to accept that a citizen can take upon himself authority as he sees fit, as long as he's really sure he's right, then our nation will be utter chaos in no time.