Ever since he found out the Master had come back to life regenerating into Missy, he must have at least considered it.

Events of Lets Kill Hitler gave him a way to do it. All he needs is a few drops of poison from the Judas tree.

No known antidote and time lords can't regenerate to escape the effects so that wouldn't help either. He doesn't even need to give her a "toxic" kiss.
Just slip it into a cup of tea or put it on a weapon and wound her.

Deep down he knows the universe would be a safer and happier place without the mistress in it.

They have a long and complicated relationship stretching all the way back to Galifrey.
Look forward to seeing their next meeting.

So you think they will turn the Doctor into a cold blooded murderer??
.. have you been watching the same show here?

Ha, as if the Master has ever needed regeneration to survive.

The poison from the Judas tree doesn't block regeneration, the Doctor was on his final life at the time, and couldn't have regenerated to escape it's effects.

The thing with the Doctor and Missy is that NEITHER seem truly capable of finishing the other other off. Now and again, they might try it, but I bet they are secretly glad when it fails.

Pretty sure it was intended to mean that it did block regeneration because firstly, I doubt when Moffat was writing series 6 he was also writing series 7, so he wouldn't have come up with the war doctor and the 'being on his final life' stuff yet, but also, more importantly, if normal regeneration was a way out of it, River would have only had to have given him one of her regenerations, not the power of all her remaining regeneration energy.

Regarding the subject of the thread, I'm pretty sure we will never see the Master/Missy die for real and not come back. Who would want to lose the Master from the show anyway?

Oh, yeah. I'm pretty sure that's what Moffat intended at the time. But it was never explicitly stated, and the fact that it then transpired that 11 was the final Doctor suggests that actually, this was the "in universe" reason.

But I still think personally that the fact that River had to give him all of her remaining regeneration's rather than just one still seems to imply that it did block regular regeneration.

Doctor has killed before he doesn't like it but he does it.

10 said to WIlf in End of TIme part 1 "I'm no innocent I've taken lives convinced others to take their own". Referring to Adelaide Brooke's suicide and Rivers death in the library.

11 admitted he wanted forgiveness for the time war in Doctors Wife. Other examples are him leaving Solomon to be blown up in Dinosaurs and nearly killing Jex in Town Called Mercy.

12 has also had his dark moments Deep Breath robot man he said " I have a terrible feeling I'm going to have to kill you " Later we saw him fall out the window. Did he jump or was he pushed by 12 ?.

How far will 12 go to protect his new friend Bill and the rest of time/ space ?. They are all his "patients" after all.

I don't see how. There was no regeneration to block.

I'm almost certain it was meant to be the case at the time, but no longer.

Like I said, if it didn't block normal regeneration, then even the doctor with no regenerations left would have only needed the power of one regeneration from River.

The fact that she had to use the power of all her remaining regenerations suggests to me, even now, that the stuff is so strong that normal regeneration isn't strong enough to save you, and that River knew that the only way to save him was to give him the power of multiple regenerations all at once.

It's ambiguous:

Is regeneration disabled because of the poison, or is regeneration disabled by the limit, and this is Moffat's way of alluding to it, without committing himself to it?