Currently, when a monster reaches 0 HP it doesn't die instantly but there is a delay of sometimes up to 2 or 3 seconds. This gets really frustrating especially at Barrows, when you see the brother reaching 0 HP you want to click the stairs to go onto the next brother. But when you do so the monster hasn't died according to the game yet, since there is no instant mark before his name on the top right screen. I'm not 100% sure whether this is a bug or not, but as far as I know it wasn't like this in the past.

Is getting a change next patch.