chose the critter, put it on the map, facing the way you want.. select it.. press SHIFT+K to kill it and select the death type.., or just press k ( lower case ) to kill it with standard death.
certain critters don't have certain death types.. for example, ant's don't explode.. and so on..
just edit the instance of the critter and set one of the transparent flags to yes...
you can make it transapernt glass, wall, red, energy and so on.
each flag makes it more or less transparent, and the energy and red ones make it yellow or red respectively..