Ok. It seems like I did understood after some experimentation with the slith 
It appears that I need to more carefully decipher the description of the skills. I remembered a story of Ulzuin from the Forgotten Gods expansion and that he was betrayed by beloved Bysmiel and thus we are talking about vengeance (retaliation) this time. When the first row of foes are attacking me then they receive fiery retaliation from Vindicative Flames but the enemies who are shooting at me or standing behind the main line of attackers are receiving retaliation damage from Ulzuin’s Wrath (Electrocution and Lightning) and also it drains their health to me. Is that correct?
P.S. I am feeling myself kinda guilty because I didn’t figure this out myself but the developers wrote it right away in the game that if you have any questions then feel free to ask at forums.