And Grace didn’t have to use her the kill the Rev-9.. The Rev-9 was already damaged. The turbine had already destroyed the liquid metal skin, the most threatening part of him. He was already vulnerable. A few more shotgun blasts, They could’ve taken him apart. If you’re going to use that, do it at the beginning when he’s fully functioning. Don’t wait until he’s on life support.
I get where you are coming from, but Grace was already critically wounded and would have died anyway. If you remember not only was she stabbed by the liquid metal portion of the Rev-9 and bleeding out, but she exerted herself far beyond her natural physical capabilities (pulling the rev-9 into the turbine with so much effort she tore her own skin.) which as shown in the past scenes causes her to 'crash'. By that point she has used the last of her anti-seizure meds in the scene prior, and was FAR too heavy for Dani and Sara to carry away to get her help at the time (carl couldn't have helped he was missing both an arm and a leg) long story short Grace was going to die one way or another, better to put her out of her misery and use her PSU to ensure the Rev-9 was fully destroyed.
Its the same argument people had with T-2 years ago, when several people would constantly state 'Uncle bob' could have just stayed with them and protected John. No he couldn't like Grace he was doomed, his own PSU was destoryed by the T-1000, and he was only standing at that point because of his auxiliary power only even usable because of the heat-sinks from the heat of the steel mill. He had maybe all of a few minutes to 'live' or function and likely would have shut down before they even got out of the steel mill, better to be destroyed before being found.