Because you think they would not assassinate someone given the opportunity?

Talk about ignoring the evidence in the books .

You're basically saying "Oh, well even though we've seen Semirhage, Moghedien and Lanfear assassinate people on-screen, that's not enough evidence to assume they would do it." Which is quite insane and illogical IMO. While it would be nice to assume that given dependable servants, the three Forsaken above actually have a good reason for NOT trusting these tasks to others: Lanfear herself directly states that she doesn't use Greymen or soulless, preferring to do it herself in TSR while Semirhage gets more pleasure of drawing it out. And if you are going to argue that Moghedien's entire methodology of hiding in shadows waiting for an opponent to stumble into her web before destroying them is not an assassination, we might as well agree to disagree on this, as well.

Notice how the listing above quite neatly includes the two Forsaken who the BRB mention as being exceptionally competent with assassinations as well as the two who've been doing this throughout the series?