Basu found out the truth about the marriage and goons. He didn't reveal any of this to prerna instead he told bajaj. He has a video of the goons confessing. Proving bajaj guilty would be difficult because basu already implicated himself in the planned accident. That makes it harder for prerna to trust him. Regardless he got the confession from the goons it could be twisted that basu forced them to say that. Out of basu and bajaj the goons have more to fear from bajaj. So when Bajaj holds all the card why instantly go for the kill?