A man who never would frequently did.
The Tenth Doctor was fond of describing himself as a "man who never would" despite the fact that he was a genocidal maniac. Ten's penchant for mass murder made this statement somewhat hypocritical, particularly when he would chastise others for wiping out entire civilisations.
Ten's unrelenting brutality and insatiable bloodlust was demonstrated in many stories, from drowning children in The Runaway Bride to torturing a family for all eternity in Human Nature / The Family of Blood. His own race (the Time Lords) didn't fare much better - after catching up with them again in The End of Time Ten had a good laugh at their plight before "sending them back to hell" by trapping them all in the Time War forever. And who could forget the time Ten slaughtered an entire city full of 11,000 innocent people?
Ten was also a fan of playing God by manipulating fixed points in time to cause others to kill themselves. In The Waters of Mars one potential companion shot herself in the face after Ten destroyed her timeline for a laugh.
Shortly before regenerating Ten was heard to remark "I COULD DO SO MUCH MORE!" referring to all the innocent people he hadn't had a chance to slaughter yet.
And let's not the fact that he became the fascist ruler of Time and the Universe in a parallel universe.
Although all of the Doctor's incarnations have been known to indulge in a healthy dose of murder and genocide from time to time, none of them derived quite so much
sexual pleasure enjoyment from it as Ten.
Except maybe the Seventh in the VNAs. Because the VNAs were written by edgelord manchildren.