Doctor Who: 10 Times The Doctor Acted Completely Out Of Character For No Reason
3. Murdering An Ogron In Cold Blood
https://www.youtube.com/watch?v=QyzLeetZ80k1972's atmospheric clash with the Doctor's arch-foes in The Day of the Daleks includes a very famous mistake. The above interview with producer Barry Letts explains all and shows the error, so this description will be kept brief.
The Doctor commits cold-blooded murder.
He pre-emptively kills an ape-like Ogron without being directly threatened.
If nothing else, this lapse shows that the Doctor had some pretty shocking tendencies towards murder even before Eric Saward decided that bloodbaths and nihilism were just what the Doctor ordered.
Just for the record, the Doctor generally tries not to murder people. He has his good days and his bad days.