Vikrant wants Bella to kill an innocent. In particular, even if there were other innocents like bhultu and pratham; he specifically wants Bella to kill Mahir. Which we all know that Mahir is immune to Bella's poison. Unless they change that of course. Even if Bella bites Mahir, he will definitely survive the bite.
Secondly, Mahir is the male lead in this show. Even if the makers are planning to kill Mahir like they did Shivangi in season 2. They are not going to do it now midway the season. They will only do it at the end of the season. Otherwise, people will start to complain and not tuning in which is bad for trp, which will also be bad for business.
Having said that though I think some very dark gloomy episodes with all the misunderstanding and frustration are on their way for us all to watch. Which is going to be not entertaining for us all I supposed.