We have heard a preacher refer to the law as worst than witchcraft and that David sinned meditating upon the law day and night.
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Basically, your preacher is saying that we should not worry about rules. There are a few problems with this:
1. It should concern us when we do wrong because these wrong doings are what caused Christ's death.
2. If they do not have good works ('fruit') by following the law, then they are probably not saved (John 15:2, James 2:14).
It is not that works create salvation (legalism), but that salvation creates works (one aspect of sanctification).
3. The Psalms say very good things about those who meditate on the law. They are described as being resilient, fruitful, prosperous (not as in gaining wealth) and do not associate with bad influences. From this, we can conclude that it is noble to meditate on the law, rather than sinful.
It sounds like you have been listening to a false-teacher. I strongly suggest that you compare his/her teachings to scripture under great scrutiny.