There is nothing in humanism per se that can define anything as truly right or wrong. Certainly there are things that we all recognize as wrong – rape, murder, etc. – which seem to be inherently recognized among civilized humans and most religions, but there are no boundaries actually codified in humanism.
Humanism is a form of specie-ism that is wide open to arbitrary definitions – for instance: Hitler defined the Arian race as human, and blacks as mere animals. Humanism recognizes all races, but apparently defines children who have not yet been born as non-human (although millions who have been killed because of this definition were quite viable, and a number who had actually been aborted have survived to healthy adulthood). Given the viability factor, this definition is totally lacking in biological legitimacy, yet it is very effectively executed as a social, political, and convenience expedient.
In Humanism, there is no line that must be crossed to go from a Mother Teresa to an Adolph Hitler.
It would be in the best interest of humanity for there to be a lot fewer people on earth – in fact, a tiny fraction of the current population would be optimum for the environment of those chosen to survive. I suppose it would be assumed that the “most intelligent” would be favored in any such proposal, but historically it is the most primitive who have lived in balance. Bottom line: Humanism could be used to justify mass genocide (ask over 30 million unborn children) – I rest my opinion.