I have only ever in my life seen TWO fictional act of pure altruism. Missy’s decision to stand with the Doctor was one of them. The other was Tony Stark in Marvel’s 2006 Civil War.
True altruism is selflessness for the good of others. Doctor Who gives us a good definition of that with this season’s definition of virtue. It doesn’t seek advantage; the truest form of goodness is when there is no hope, witness, or reward.