Perhaps an AltInfDeath=# tag on weapons for animation to play when target has NotHuman=yes. If not set default is AltInfDeath=1 for the target, maintaining compatibility with what is the status quo. This, in conjunction with additional Infantry deaths which are now possible gives the optimum solution IMO.
Edit: It also means that the Genetic Mutator could generate an alternate unit depending on if its target was human or animal.
Edit: It also means that the Genetic Mutator could generate an alternate unit depending on if its target was human or animal.