The Satan of the Bible is quite real. The Devil and devils of Renaissance imagination (cf. Dante, Botticelli etc.) are not. Folks often mistake the second for the first.Quotei have noticed how many so called religious people actually appear to be quite evil...
There is no adequate non-religious/secular basis for declaring anything or anyone "evil" in any meaningful/non-trivial way. Which is to say that the secular person must borrow from a religious world view like historical orthodox Christianity (i.e., "mere Christianity) in order to make meaningful value judgments.
Whoa ho ho! So the basis for any value judgement is religious? Or do you just mean it's the basis for any judgement of "evil"?
The apparently non-religious or secular writer I quote above makes a value judgment (i.e., he ascribes the property/quality "evil" to certain "religious people") which cannot be or is at least unlikely to be true except upon a religious foundation, which is ironic since the point apparently escaped his notice.
Said otherwise, if the non-religious or secular worldview the writer writes from is correct, at least with respect to it's moral philosophy component (i.e., most likely some form of moral relativism, such as private subjectivism), then it is false that religious people really are evil. Rather, he simply does not like them, their character or their conduct. Non-religious moral philosophies invariably reduce right and wrong to personal preference, or something even moral trivial such as non-cognitive emotive utterance, since they cannot ground real right and wrong (i.e., moral facts such as "it is always wrong to abuse infants for one's own amusement). But since there are moral facts (e.g., "Hitler was absolutely wrong to kill Jews for being Jewish"), no non-religious moral philosophy or worldview can be correct.
More to your questions, this bit is not about whether the moral judgments people make actually and always proceed from religious conviction but is instead about whether a secular point of view can even support a thing like true moral judgments in the first place.