I have just recently killed an epic creature in creature stage & have not got the epic killer badge.

It's not a badge it's an achievement and did you use any cheats at all in creature or cell? If you did you can't get any achievements on that save file but you can still get them on other saves as long as you don't cheat

Remember that you have to be the one to get the killing blow too to receive credit, which makes using your nest mates and pack a bit more tricky :P

I have not used any cheats before & I was the one to deal the finishing blow