From Creatures Wikia
All creatures will die over time. Some die of old age, some die from stillbirths.
Most creatures will most likely die if they lack glycogen or life/ageing chemicals. Norns can also die if they are in too much pain, succumb to fatal sickness, get killed by another creature or drown. Creatures that have an immortality mutation cannot die from old age, but may still be able to succumb to the other deaths stated.
Death in Creatures 1
When Norns/Grendels die in the first game, their bodies will stay for quite some time before finally disappearing. Even though a Norn is dead, the game still retains its files, which can take up a substantial amount of hard drive space if they aren't deleted.
Death in Creatures 2
Death works slightly different in the sequel; Norns, Ettins and Grendels all now have a death pose, and after around a minute, Norn corpses will slowly begin to ascend to the sky, or "go to heaven".
If the corpse is disturbed during the process of ascending, such as being caught in a lift or on a vehicle, it can alter its flight path, and the body may circle Albia indefinitely. This can be fixed by using a spring to launch the body into the sky.
Unlike the original Creatures game, Creatures 2 will delete the files of any deceased Norns, freeing up hard drive space.
Death in Creatures 3/Docking Station
Death in C3/DS is similar to Creatures 2, but rather than ascending into the sky, Norns, along with Ettins, will disappear in a puff of smoke. Grendels on the other hand will melt into a puddle of green slime and disappear.
In Creatures 3, Creatures can also be killed by being sucked out of an airlock or being eaten by piranhas; when sucked out of an airlock the creature will explode into a cloud of bones, whereas when eaten by piranhas all that will remain from the Creature will be a few bones.