Not a nice story, especially to Rose. This is how I see season 4 Rose. Most of you will not like. Consider yourself warned.
Stopping the End
The kiss had meant nothing. Nothing. It was an in the moment thing, and as soon as she realised that the other him had left in the TARDIS, she had dumped him without a second thought.
Rose Tyler, the woman who wanted nothing more than the version of the Doctor he was, had dumped him because he wasn't what she wanted.
What she wanted included a TARDIS. What she wanted included a man who could, if he didn't die, stay the way he was for her forever. What she wanted was to go back again, and get what she wanted. Nothing else mattered to her.
What she wanted was a fairy tale she could never have.
Why could she never see that?
Why does everyone else have to suffer because of it?
To start with, he decided that destroying the dimension cannon would be the way to stop her. She had screamed at him for months afterwards, until another one was built. The second time around, he let her use it.
As the cracks between the dimensions grew, things kept on leaking through.
Let her see what she was doing. That it was a foolish idea and was destroying everything.
She was too deep into her obsessive need to get back, she refused to see anything else.
An earthquake hit and her flat was destroyed. She hadn't even blinked.
Her mother and this world's Pete, as well as her little brother were killed in the first flood. She hadn't cared. She had shrugged off any feelings for them she may have had a long time ago.
He could feel it happening now, the realities crumbling, and he had to stop her or everything was dead. Two universes, maybe even more, would be destroyed. And with them the one she was trying so hard to get back to.
In the end, it was the only way to stop her. The huge gun she was so fond of was in his hands. He pointed it at her and pulled the trigger. It destroyed the machine behind her as well.
He had dropped that gun in favour of the small handgun he'd been carrying around for the past few weeks, waiting for the right time.
Now was the right time.
He turned it to his own head and had the brief thought that at least he had saved everything, the rest was up to the other Doctor, before he pulled the trigger.
The only thing he had been able to feel in his last few moments of living, was the deep relief of knowing that it was finally over.