One of the main causes of the Biblical disease tzarass was evil speech . Part of the repentance program for a metzora (suffering from tzarass) was that he would have to “sit alone” . But surely he should instead be doing the opposite: learning how to integrate into society?
Furthermore, we are told that “on the day of his purification he will go to the Kohen” (for instructions of his repentance program) . But, the question is asked,
Why is it referred to as “the day of his purification” if a series of repentance is specified afterwards ? 
Finally, forbidden speech is generally called ‘lashon hara’, literally translated as ‘evil language’. But ‘lashon hara’ can include various types of speech even if they are not actually evil. Indeed entire volumes have been dedicated to discussing them!  In fact, under some circumstances even saying something good could be within the category of ‘evil language’!  So why then is forbidden language called ‘EVIL language’?
A middle-aged couple had been married a long time but had still not merited to have children. They tried everything and were almost ready to give up hope, but as a last effort, the husband – who we will call Ploni – sought the advice of a Rabbi. The Rabbi asked him if perhaps someone was bearing grudge against him.
Ploni nearly fainted. Indeed there was somebody whose forgiveness he had to seek. But would he forgive him after all these years? Would he even be able to find him after all these years?
It took significant amount of extensive, even fanatic research, but eventually Ploni found the address of his old classmate. It was in a rundown part of town, and the house was no more than a hovel. Looking through the broken window he noticed what appeared to be the bedroom: there was a single bed in the corner, the rest of the room a complete mess. He knocked on the door and waited a while. He knocked again, and then began thinking of the contrast to his own spacious home, well kept by his faithful wife. His thoughts were suddenly interrupted by a movement of the door.
“What do you want?” a broken voice called to him.
“I am an old friend of yours. Can I come in to speak with you?”
The door closed.
It took a number of efforts until the door finally opened and there stood a broken man in tatters. Could this really be his old classmate? The man turned to clear the floor and make a pathway for his guest – and there was his limp. Yes – it really was his classmate.
Ploni profusely apologised for all the pain that he had caused. In tears he related how he had also been suffering in his own life, not meriting children, and what the Rabbi had told him. He begged and begged until he was finally forgiven.
A short while later Ploni and his wife received their long awaited good news – but Ploni never forget the effect of his words upon his classmate.
Who can possibly predict what his actions and words will ultimately cause?
“Sitting alone”, the metzora not only has a chance for general reflection of his evil language, but it also gives him a chance to taste what he might ultimately be causing the person he spoke about. Even if his words do not physically isolate a person, evil speech can at least emotionally ‘isolate’ them to some extent.
Therefore when the metzora simply leaves seclusion, he has hopefully learned and internalised this message, and is already (largely) considered “pure” once again.
‘Evil language’ might not be ‘evil’ words per se when spoken, but the results could ultimately be devastatingly evil – and so Divine wisdom dictates that it is always called ‘evil’.
Have a pure Shabbos,
 Erchin 15b
 Vayikra 13:46
 Ibid. 14:2
 E.g. Ibid. 14:23
 Divrei Yoel Vol. 5, p. 369 (q. 6) and p. 376
 E.g. Shmiras Halashon; Chafetz Chaim
 E.g. Chafetz Chaim, Hilchos Lashon Hara 5:4