The only result that matters is the final one. Regardless of whether they reroll, have a + to hit, or a - to hit.
So you can reroll a 1 for Overheating to avoid the death; if you have a +1 to hit and roll a one, it doesn't kill the model as the final result is a 2; and if you have a -1 to hit, they will die on a 2 as the final result is a 1.
I've killed an entire unit of Plasma Inceptors overcharging against an Eldar flyer because they were dying on 1-2, and I had no rerolls nearby; and I have saved countless marines dying from Plasma because I run Dark Angels with an innate reroll.