The OP makes the odd argument that because evil can be explained, it doesn't exist. IF we very narrowly define "evil" as a root desire to cause harm and suffering for its own sake, then of course an explanation of "evil" acts will conclude they are not due to such "evil", because behavioral motives are rarely unicausal and there is always a more distal motives at work. That means "good" doesn't exist either, because no one is good just for goodness sake (Fuck off, Santa). But that only means that that narrow definition of evil does not exist. There are other reasonable definitions that might exist. For example, a person with so little empathetic regard for others that they make no minimal effort to causing unnecessary pain and suffering to others. This includes people with "wrong" beliefs about what is necessary or a threat, etc.., because often those beliefs are highly irrational and the results of a failure to engage in minimal effort to ensure that they are valid.
IOW, evil is an type of active negligence where you act to cause self-serving but needless harm, not necessarily because that is your ultimate goal, but and don't care enough to bother trying to avoid causing it.
Also, the OP it makes a false equivalence between an action being motivated by the personal it yields, and an action being necessary for survival. Few specific acts are required for survival. We must eat and protect ourselves from threats, but there are countless ways to do these, thus making any particular way unnecessary, and many actions not even relevant to these goals. My definition above make it more clear that a failure to make an honest effort to distinguish necessary from unnecessary harm caused by one's actions is itself evil.
All of which is well and good; but leads to 'evil' being synonymous with merely 'psychopathic', which seems to me to be a less extreme descriptor. I would assert that the former label implies that little or nothing can be done, short of removing the 'evil' individual from society (either by lifelong incarceration, or death); While the latter implies a pathology that, in principle, may be amenable to treatment.
I hear people say "He is evil, and must be killed", when perhaps a better statement would be "He is a psychopath, and we must protect society by placing him in a secure psychiatric hospital until we are able to cure him".
Only if 'evil' refers to something more fundamental than a mere pathology is it a valuable concept. Its origins in the belief that a God imbues (or fails to imbue) each person with fundamental traits that define all of their actions, strongly suggests that 'evil' is as valuable a concept as 'soul', 'angel', 'demon', or 'God' - ie of no use at all outside fiction.