Blowing up a solar system was destructive, yes, but only the one research assistant died in that one iirc. So I’m not sure if that event gives McKay the deadliest scientist crown.

He did re-enable the attack directive of the replicators which led to countless human and wraith deaths.

There’s also that time when McKay started ripping a hole in an alternate universe’s spacetime that would have destroyed an entire universe.

Beckett’s contributions to the Hofman drug might make him deadlier than Carter.