There are 2 ways to get that exclamation mark:
1) You make/get a exclamation mark model like the Blizzard one, but with Team Color skin, then you apply the color of the owner of killing unit to it
2) You make the Blizzard exclamation mark model the model of a dummy unit with flying height to make it float over the head. Then you change Vertex coloring of the unit to the color of the owner of killing unit. Vertex colors go in Red/Blue/Green percentages, so you'll have to use If/Then/Else commands 'If Player color of Owner of Killing Unit Equal to Red, Change vertex of UNIT to 100,0,0, else If Player color of Owner of Killing Unit Equal to Blue, Change vertex of...', etc. On top of this, the exclamation mark is not an attachment, so it will not move along with the unit when the unit is moved.