Usually he avoids killing, and tries to give everyone a chance for redemption—but sometimes they don't take that chance. For example, the Tenth Doctor spared the Sycorax leader's life after defeating him, but when the leader tried to attack him after surrendering, the Doctor hit a button which sent him falling to his death.
And sometimes, especially in desperate straits, he has no choice. The most blatant is, of course, his reluctant willingness to use the Moment to wipe out all of his own people and the Daleks, because the only alternatives would be to let the Daleks conquer the universe or let Rassilon carry out the Final Sanction, both of which were far worse. For a long time, he believed that he had destroyed the Time Lords along with the Daleks but he eventually learned that he'd found a way to save Gallifrey & the Time Lords (in circumstances that prevented him remembering it) -- but he did destroy almost all the Daleks, albeit not in quite the way he thought he had.
He has occasionally killed others in combat. For example, the Third Doctor killed an Ogron during a firefight involving UNIT & Dalek forces at Auderly House.
He seems to be much less concerned about tricking enemies into killing themselves, especially if a less evil/arrogant/greedy enemy would not have been tricked. He also seems to count Cyber and Dalek life less than other forms—this may be partly just because Cybermen and Daleks are more likely to kill themselves out of their own greed, but he's quite often destroyed them directly.
There are also a few cases where he seems to have killed unnecessarily. For example, in "The Two Doctors", the Sixth Doctor attacked Shockeye with a cyanide-soaked handkerchief and suffocated him with it. This is to this day one of the most talked-about scenes in the history of Doctor Who, as it shows a side of the Doctor we never expected to see and probably will not see again. The Eleventh Doctor killed the space-marauder Solomon in "Dinosaurs on a Spaceship" by deliberately leaving a device on his small spacecraft to attract incoming missiles.
Finally, different incarnations seem to have different codes, or at least to interpret and/or apply their codes differently. It's hard to imagine the Second Doctor suggesting that Za be killed because he was inconvenient the way the First Doctor did, or the First shooting down a Cybership with an X-ray laser the way the Second did. Similarly, the Sixth Doctor would never had laid the Hand of Omega trap for Davros, but the Seventh would never have killed Shockeye in cold blood.
- ↑ "The Christmas Invasion"
- ↑ "The End of Time"
- ↑ "The Day of the Doctor"
- ↑ "Day of the Daleks"
- ↑ "Remembrance of the Daleks", "Silver Nemesis", "Flesh and Stone", among many other examples.
- ↑ "The Wheel in Space", "A Good Man Goes to War", etc.
- ↑ "An Unearthly Child"
- ↑ "The Wheel in Space"
- ↑ "Remembrance of the Daleks"
- ↑ "The Two Doctors"