I think it shouldn't matter if it is an existing brand, as long as the graphic here does not dubiously resemble the logo (Grenade is a common noun), here you are not trying to use it as a brand name....
Having a concept is stronger than just pretty pictures most of the time I would say. Your grenade is nice, done before and lacks exploration.
Think about how you can enhance what a grenade does or is, and tie in some illustrative elements that speak to the grenade's purpose or misuse, for example.
About my design