With regards to the OT:
I think the word 'evil' stems from our recognition that something is terribly wrong with life. This is one point upon which most people, religious, a-religious, and otherwise, do not differ.
We know that there is something wrong. We have generated answer after answer-- way of life after way of life-- in an attempt to fix either aspects or the totality of that wrongness, and in doing so we attribute blame. When an entity or event comes bearing that which we feel to be bad (death, the corruption or destruction of what redeemed us and made our lives worth living, or some other horror) we often attribute evil to it. Particularly if it acts intentionally or knowingly.
Why? Because it embodies, heralds, or enacts what is so wrong with this existence of ours.
To me, though, a person is truly evil-- as I am truly evil-- when they willingly partake in and increase that weight of suffering and intolerable circumstance in the life of others, without doing so in the perceived service of the mitigation or eradication of evil. I do so because I have taken a decision to not reflexively react to the pain which has, via reflexive reaction, generated so many strictures upon the being of humanity. I do so because I feel that only once we overcome such a bifurcation-- which most who speak of evil's metaphysical subjectivity have not even come close to overcoming-- can we see with the eyes of Gods. And, differently, because learning to manipulate the content and influence of that duality upon individuals and groups is vital to really understanding and changing them.
But, mostly and primarily, not because.
So: a person is evil when they are of evil. The rest... they tend just to be humans living their lives and enacting their desires in a way which causes discomfort or pain to others, without necessarily intending to. They may partake in evil, or presence it, but I would not say that they themselves are evil.
True evil... cannot be constrained by your reasoning, the concepts you have established to protect you from the brutality and dynamic viciousness of life, nor by your pleas to good. True evil machinates from beyond cause and expectation, and from beyond the reflexive slavery to the good which is so many people's definition of 'humanity'. When one enters into true evil-- seeks it, and seeks to know it via embodiment-- one enters into a realm beyond former reason, beyond former conceptual protections and arrogant 'knowledge', and thus beyond one's former limits. By entering into true evil, we overcome the bifurcation that we have been.
True evil is a gateway to the cosmos, unknown and unconstrained: the cosmos humans have sought to protect and separate themselves from for so long. It is a gateway from what has been the 'human' to what is beyond the psychic constructions which maintained and constituted it. It is, to finish, what human hubris sets itself against, and what it is inevitably destroyed by.