How evil it is to tell-tale, get a gist of it from the fact that the Holy Quran has condemned a tale-teller. It is stated in part 29, Surah Al-Qalam, Ayah no. 11:

هَمَّازٍ مَّشَّآءٍۭ بِنَمِیْمٍۙ(۱۱)

Translation from Kanz-ul-Iman: The excessive slanderer, going about excessive slandering.

Dear Islamic brothers! The way Holy Quran has condemned a tale-teller, similarly, blessed Ahadees also condemn the tale-tellers. For admonition, let us go through six blessed sayings of the Holy Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم in this relation:

Six sayings of the Holy Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم in relation to tale-telling

1. He صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم said: Backbiting and tale-telling cut Iman [faith] in the same way as a shepherd cuts a tree. (Attargheeb Wattarheeb, vol. 3, pp. 405, Raqm 4362)

2. He صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم said: The most disliked among you in the court of Allah Almighty is the one who creates conflict and discord among Muslim brothers by telling tales to one another to create enmity among them. (Musnad Imam Ahmad, vol. 6, pp. 291, Raqm 18020)

3. He صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم said: The best people of Allah Almighty are those when you look at them, they remind you of Allah Almighty. And the worst people of Allah Almighty are those who roam around to tell-tale, create separation among friends and keep looking for the flaws of flawless people. (Musnad Imam Ahmad, vol. 6, pp. 291, Hadees 18020)

4. He صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم said: Those who talk ill on the face, take faults out behind one’s back, tell-tale and find flaws in flawless people will be gathered in the shape of dogs (on the Day of Judgement) by Allah Almighty. (At-Tawbeekh wat-Tanbih, pp. 237, Hadees 216)

5. He صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم said: Tale-teller will be punished in his grave before the Hereafter. (Sahih Bukhari, vol. 1, pp. 95, Hadees 216)