That doesn't appear to be what the writers are going for, luckily, and anything that can instant-kill a regular person, like getting his head popped, would do the same thing for Null.
Null can cheat with his death field, but I'm guessing that is proportional to the amount of energy he can absorb at once; too little, and he's a corpse, as he wouldn't regen fast enough. Useful against hordes of human mooks in melee, next to useless vs getting haymakered punched by Juggernaut.
Honestly, if we're talking about what powers Null could manifest, I'd be more excited if Null develops a weak healing field power just because of how rare that power is in Marvel and how useful that'd be to the X-Men. Seriously, even if it just boosts natural healing to below Spidey levels when in range, that'd save so many people's lives.
Combine it with the death field, and Null can zap illnesses and then heal the damage (yes, I am talking about the legacy virus).
It'd mean Null would have to stay out of the fight more often than not, but a support role isn't bad.