This is one of the "Unbound" stories by Big Finish Productions. The "Doctor Who: Unbound" BBC radio series featured The Doctor in alternate realities, i.e., "what if...?" story lines. In this story, what if The Doctor believed the end justified the means? This really isn't some far our idea as the producers must have thought, considering past story lines on the TV series. The Doctor has even used genocide in the past on the TV series (both old and new) when he felt there was no alternative. He even destroyed Galifrey and the Time Lords when he saw no other alternative. So he does believe the ends justifies the means depending on the situation.
This story has a bit of a "plot twist" at the end which I didn't like at all. The Doctor, in order to stop genetic mutations from finding their way into he human population, kills who ever he feels he must...and this time it's 2 or 3 people, not entire planets. The Doctor comes off as being a bad guy, which is a bit too much of an alternate reality for me, and probably most other Whovians. At the end of the story, the companion Ruth, kills the Doctor, the Doctor regenerates, and she kills him again, wondering if she has enough bullets to finish him off for good ("I wonder how many lives this b-----d has?"). Not a very happy ending! The production values and writing were good, for which it gets two stars instead of one, but the story itself was bad.
However, the other stories I've heard in the Unbound series are far better than this one. I would recommend "The Masters Of War", (which I'd give 5 stars) which features the Daleks...and I won't spoil the rest of that one for you.