Evaluating whether an act is right or wrong is a subjective process, based on one's experience and set of morals. Shakespeare tells us that Friar Lawrence hoped to mend the feud between Romeo's and Juliet's families by marrying the young couple. Shakespeare suggests that Friar Lawrence accomplished this, but the outcome proved more tragic than Friar Lawrence imagined. The families were mortified by the deaths of Romeo and Juliet, but at the end of the play, Shakespeare insinuates that the suicides might stop the petty arguments between the families. Did Friar Lawrence want to create such a tragedy? Not likely. Rather he was motivated by his own personal desire to create peace.
The same might be said about Friar Lawrence's giving Juliet the sleeping potion. This was also a good plan gone bad. Yes, his ideas were deceptive, but he weighed the so-called sin of deceit against his hope for a positive outcome. Should he have more carefully calculated the things that might go wrong? Possibly. He might have had a backup plan that could have saved the young lovers, such as making sure that his message was delivered to Romeo explaining that Juliet was not really dead. But this could only be judged as an oversight not as an evil act. In other words, Friar Lawrence's head might not have been in the right place, but his heart was.