Why did the Hands of Blue use a “sonic device” to kill people?

Question

While the use of that weapon does a good job establishing these are seriously evil people, it seems to have nothing but disadvantages compared to something mundane like a silenced pistol:

• The victims scream, very loudly. In fact, this seems to be the only reason Jayne started listening to River and ran away in time.
• The "massive bleeding from every orifice" (as the wikia puts it) makes it very obvious a non-conventional weapon was used to anyone who stumbles across the bodies later, so you don't gain any secrecy from using it.
• It seems to take several seconds to kill anyone with it.
• It's not clear what the weapon's range is, but considering how closely they held it to their victims I doubt it's that long.

The dialogue seems to imply that their killings are intended to destroy anyone's memories of interacting with River, but surely a simple bullet to the head would accomplish that as well? As far as I know there wasn't any evidence in the series that the Alliance could extract memories from dead bodies or anything like that.

Answer 1

It probably can kill multiple people

From the original script for Ariel:

The SOUND OF SCREAMING.

INT. HOSPITAL - SECURITY SUBSTATION - HOLDING AREA

Jayne, Simon and River react to the continued SCREAMING... what the hell was that?

RIVER Hell is coming to breakfast.

Simon looks at River, not certain what that means, but certain he doesn't like the sound of it.

SCREAMS come from beyond the door. First, just the one. Then more -- all in the same agony as McGinnis when he died.

Later:

INT. HOSPITAL - SECURITY SUBSTATION - RECEPTION

ON MC GINNIS

As we left him, very much dead. Then, CAMERA DRIFTS through the reception area... FINDING another Fed. Also dead, also bleeding out of many orifices. CAMERA CONTINUES TO DRIFT... finding another Fed. Same thing. And another. And another. Every Fed we've seen lies dead in a pool of blood.

CAMERA CONTINUES TO DRIFT... finding the Blue Gloved men walking through the carnage, completely unaffected. They walk up to the separator door, walk through

The phrase "first, just the one, and then more" suggests that multiple people were screaming at once, which is consistent with the sonic weapon affecting an area, with people nearby being affected first. Note that

They killed several Feds (at least six). With only two Hands of Blue and one device, unless they could somehow restrain the Feds, they might find it difficult to kill them with the sonic Device one at a time, if it had a short range. This suggests that they were able to kill multiple people at once with the Device.

From the sequence of events in the episode, it would seem that the person nearest to the Device was affected first (i.e. McGinnis), and that the other Feds were affected later.

They probably didn't need to hold it near their victims for it to work; it's simply that it took effect most rapidly in that case (as evidenced by McGinnis being the first to die). As indicated here, their odd movements and unusual affect makes it quite likely they were not at all biologically typical (i.e. cyborgs or robots). An area weapon that the user is immune to makes a very effective tool.

_{Note that the script dialogue has some differences from the actual filmed version.}