Evil is ultimately this: it is the undervaluation of the well being of others. Actions which partake in this undervaluing relationships are evil actions. People who tend towards these actions are as evil as the strength of those tendencies. Institutions which engender such people are evil institutions.

Wanton violence is evil. If a gang drives through a village and takes pot shots at the villagers, they are evil since they pay no consideration to the villager's well being.

Negligence is evil, for instance letting someone drown because you are too lazy to row your boat over to rescue him. It overvalues convenience against the loss of the drowned.

The thief is usually evil as the thief values his gain over his victim's loss.

Racism is evil, it systematically undervalues an ethnic group, with negative consequences for their well being.

Slavery is evil, as it overvalues the benefit to the slave owner of the stolen work, and undervalues the freedom of the slaves.

War is almost always evil, it undervalues the death and suffering of all the victims relative to the objective, where the real objective is often just making money.

The evil of abortion is as ambiguous as the value of the fetus in each of its stages is ambiguous.

On the contrary, for seculars homosexuality is not evil, as it does not in itself undervalue the well being of others.

I can think of no instance of what is called "evil", in the secular sense, which does not participate in this relationship of undervaluing.

Evil is a relationship, that of indifference. If follows that since relationships are real, and most importantly patterns of relationships are real (evil is instantiated everywhere, in all kinds of ways), that therefore evil is real.