Lieutenant Torres had broken off a piece of metal and attacked the Vihaar at the end of this episode and from what is shown she had killed him prior to the vessel explosion. It's very well known that Starfleet isn't in the habit of killing life forms. Even if the Vihaar was in fact dead already it doesn't justify what she did. She should've been reprimanded for her actions when she returned to the ship. However.... Torres had no alternative but to stop the Vihaar by any means necessary otherwise many more lives would have been lost. She wasn't able to use her phaser to stun him because it would have been too dangerous to do so. But it was not made clear whether she killed him or just rendered him unconscious. The creature was dying anyway.