Why do you think Macbeth kills the chamberlains if this wasn't part of Lady Macbeth's plan?
Concerning Shakespeare's Macbeth, several possibilities exist for explaining Macbeth's killing the grooms even though doing so wasn't part of the original plan.
First, Macbeth is already sleep deprived, as he will remain for the rest of the play as far as the reader can tell. Clear thinking is not an attribute of someone who hasn't slept. That's a bit of a stretch, though, when one considers how quickly he rationalizes an argument to excuse himself for killing the only possible witnesses, when he answers Macduff's question a few minutes later.
Second, Macbeth may have just reacted to the moment. An initial reaction against letting the grooms deny their involvement in murdering Duncan is excusable. Their denials could have led to other suspects being sought.
Third, Macbeth may have been trying to demonstrate his independence, as concerns his wife. She manipulated him into going ahead with the assassination, and from this point of the play on, Macbeth shuts her out of all decision making. He rules alone, from this point on. Killing the grooms may have been his first independent act.
Whatever the reason, Macbeth makes a mistake here. His wife is the planner of the family, and he should have let her continue to act as such. His killing the grooms makes Macduff suspicious, and Macduff is the man who will ultimately cut off his head.