It kind of brought up something I'm curious about, but, what is the Jewish perspective on evil and good, I've read some websites, but, am still a bit confused. Does Judaism believe evil doesn't exist, as some religions and Philosophies claim, does evil come from God?, from a Jewish perspective, are there spiritual beings who can be classed as evil (e.g. demons, etc), etc.
So, I would say that Judaism is not technically dualistic in the sense that there is not an independent power of embodied evil, which is in some way a rival to God, like the Devil in Christianity.
We say that God, as sole originator and creator of all, is ultimately responsible for the existence of evil. If you check Isaiah 45:7, it says
yotzer ohr uvoreh choshech, oseh shalom uvoreh et ha-ra, ani YHVH oseh et kol eileh. "I form light and I create darkness, I make peace and I create evil, I Hashem do all these things." But that does not necessarily imply that God created an ultimate personalized embodiment of evil, like the Devil. That means that evil can be traced back, ultimately, to things which are not, in and of themselves, evil, which are God's creations.
Now, when we speak of evil, we have a tendency to include things which are not, technically, evil. Evil demands (as they put it sometimes in the law) malice aforethought. It requires intention. The intention may not be "to do evil." It might be-- and usually is-- to do something which the evildoer has convinced themselves is good, but which the rest of us can clearly see is not. But it requires thoughtful action. Which means that we cannot accurately label such as natural disasters, plagues and illnesses, accidental misfortunes, and natural death from old age, to be truly evil. Destructive? Sure. Unfortunate? Often. Harmful? Yes. Undesirable? You bet. But not evil. These things result from the natural processes of entropy, chaos, and competition of species which are necessary to the proper functioning of the universe. Can we "blame" God for them? Sure, God created the universe, and for whatever reason, He created it to require those processes to function. But it wasn't personal. It's not like, if my grandfather dies, that means that God, in creating the universe, said, "I better make sure and create cancer, because one of these days, Max is going to come along, and I have got to get that S.O.B."
But people can make evil. We do it all the time. And that's real evil. That is something of which, I am sure, I need give no examples. We are all familiar with them.
Presumably, if there are other kinds of beings in the universe that are self-aware, reasoning, and possessed of some kind of free will (and I believe that there are) those beings would also be capable of evil.
The traditional Rabbinic and post-Talmudic texts speak about demons, sometimes. They are portrayed in various ways, according to various kinds. But one thing is a constant: they are not in rebellion against God, not in the same way that Christian fallen angel demons are said to be. In our literature, demons will obey the Name of God if wielded against them, and acknowledge God's authority over everything, including themselves. They are portrayed more as beings utterly devoted to greed, lust, desire for power in the physical world, with a universal propensity for taking a perverse pleasure in mischief, and in the misfortunes of others. In short, little different in character and temperament than many lousier people, including perhaps some politicians we might name.
Later, in the Middle Ages and early Renaissance, demons were seen as being rather worse: they sort of "fed" off of the suffering or the sorrow of living people, and so were likely to do all kinds of mischief for the purpose of feeding off the resultant unhappiness of their victims.
I think in the medieval period, there was something of an element of dualism to this kind of demonic thinking, in that demons at that time-- due to the influence of Christianity-- were informally seen to be working at odds with God. Formally, of course, such an idea did not enter Jewish dogma, simply because it is ultimately incompatible with pure monotheism. But this was folk belief we're talking about: superstition, not real religion.
Personally, I think that if there are demons, they are likely to be either not truly evil, but merely naturally occurring aphysical spiritual entities whose instincts run to a kind of narcissistic indulgence of curiosity, personal whim, and psycho-emotional appetites that is effectively productive of chaos. Or, they are actually the spiritual essences of human beings who, through personal greed focused through black magic, have managed to wrench their souls out of the natural process of death, rebirth, and onward motion to other planes of existence, in order to remain here in an aphysical form that, through the drawing-in of other spiritual energies, can become powerful enough to manifest actions in the physical. These creatures would, having once been human, legitimately be evil. This, though, I confess is my own opinion and conjecture, and not any authoritatively stated opinion in Jewish tradition.
As for the Sitra Achra, that is not a mainstream belief of the Rabbinic tradition per se, but rather a product of Zoharic Kabbalah (and, to a lesser extent Lurianic, although the ARI z"l's universe is really completely different in structure from anything previous in Judaism, even in Kabbalah; and in his world, there is little room for angels or demons. The closest you usually can come is certain understandings of the complex and different energies released in Shevirat Hakelim, the Breaking of the Vessels, but it's really kind of a whole different thing). There are complex understandings of precisely how the Sitra Achra works, but the quick and dirty version is that the Sitra Achra, as opposed to the Sitra d'Kedusha (the Holy Side of things), is that side of the emanations of God's energy which is furthest from Ein Sof (God's true and Infinite core of being). It is only because of the Sitra Achra that free will can exist (the Zohar would tell you), in that only in the influence of the Sitra Achra can any creature make choices that go against God's will. But the Sitra Achra, in true Kabbalistic thought (as opposed to how the term is sometimes employed in folk supersitition) is not an embodiment of evil, or an alignment of evil forces, or any such supernatural evil power. Rather, it's something like a "God-free zone," that is (depending on one's interpretation of the Zohar) either empowering of beings to exercise free will to do evil, or it is engendering of evil in such beings (in other words, sort of the ur-force of the yetzer ha-ra).
That said, I must confess that I have very little inclination to believe in a Sitra Achra. I think, first of all, that if God is omnipresent, there can be noplace where God's presence is less. But in any case, I think the idea misses the point. There is no need for a Sitra Achra, because it is God's will that we have free will. There is a difference between God's will and what God would prefer from us. God's will is that we have free will. That God would prefer that we use that free will to choose to do right does not negate that He wishes us to have free will. In that sense, even if we abuse that free will, and choose to do evil, we are still following God's will. We are merely ignoring His preferences.
Anyhow, if you'd like to explore Jewish beliefs about the supernatural more, I cannot recommend highly enough
Jewish Magic and Supersition: A Study In Folk Religion, by Joshua Trachtenberg
. A very well-researched book, and an excellent place to begin one's education in Jewish magic and folk mysticism.
It makes me feel special 
