Not the “killer”, since it was him who alerted the cops. They were gathering evidence against the guy who HIRED that killer. Probably by wiretapping their conversation when the killer went to collect the second part of the payment
I can’t speak to this case in particular, but depending on whom the investigators believe to be behind the killing, this might absolutely be the best option they had.
Yes, there is a flaw. You haven’t taken into account how selective is application of Russian laws. Yes, FB/Twitter/whatever are required by law to comply; no, that absolutely does NOT mean that if they don’t, Roskomnadzor would block them too.