Having immediate grenade explosions sounds like it'd just be a crutch for people who lose melee fights, while also having griefing potential if the damage can spread to teammates (which seems to be the implication). I'd say have the explosion come with a delay so enemies can at least get back and reduce/avoid damage. That, and/or perhaps just make it a basic elemental explosion of your subclass type rather than an actual grenade type (we shouldn't encourage Titans suicide rushing Supers for instant suppression, Warlocks getting chain lightning explosions or mini-nova explosions, Hunters getting...whatever annoying grenade they have, etc). A martyrdom exotic seems interesting on paper, but it should be reasonable to fight against.