Doctor Who: 9 Ways To Kill The Doctor Permanently
5. A Mid-Regeneration Gunshot
When the Doctor regenerates, their body is lingering somewhere between life and death. Whatever injury caused them to regenerate is pushing them towards the dark, but that powerful golden energy is attempting to bring them back towards the light.
So what happens if you interrupt that process and push it more towards the dark?
Well, look no further than the Series 6 opener, in which the Eleventh Doctor - midway through regenerating - is shot with a laser blast. He's unable to complete his regeneration and the gunshot halts it abruptly, which causes him to fully die.
Now, granted, this version of the Doctor is actually a robot disguised as the Doctor, but River's line "his body was already dead... he didn't make it to the next one" indicates that the outcome would've been the same, even if the Doctor was real.
Presumably, any fatal injury that's sustained in the middle of a regeneration will produce the same result, which is probably one of the reasons why the Doctor normally chooses to regenerate from the safety of the TARDIS.