Humanism doesn't claim people are innately good or just.
It recognises we are equally capable of both good and bad decisions & actions, and we need safeguards in place to promote good judgement & actions while minimising bad judgement & actions along with the damage they cause.
Humanism is based on the principles that individually and collectively:
Human life is valuable;
We have control over, and responsibility for, our actions;
Critical thinking and evidence based analysis provides the answers that will allow us to progress most effectively.
It follows from these that a cooperative, inclusive approach to society is the most beneficial for humanity. Right and Wrong are determined by the relationship between any potential happiness and/or suffering resulting from our actions.
Human generated atrocities are examples where an individual or group were prepared to pursue actions to further their own benefit, regardless of the suffering of those they impacted. We collectively attempt to avoid such situations by limiting the power held by any one individual or group; promoting leadership ideals of selflessness and concern for all under their power; and employing punishments for those whose actions demonstrate disregard for the suffering of others.
This contrasts with a religious perspective that there exists a divine being who is the arbiter of what is right and wrong, and if you do what they say then that being won't punish you. Atrocities are the consequence of non-compliance by you, or those around you. You cant hope to avoid it because your suffering will be part of his plan that no-one can understand. However, you will be rewarded for your compliance after you are dead and the ones that committed the atrocities will be punished after they are dead (unless they ask that being to forgive them for the atrocity before they die, in which case they'll be rewarded the same as you - and that is considered fair because His plan is unknowable).