In Western traditions, evil is typically seen as personified into a being with a power similar to the power of good, locked in eternal battle over the souls of humans. However, this concept did not exist in early Judaism, which held Yahweh responsible for both good and evil. One of the oldest Hebrew texts, Isaiah states in 45:7:
I form the light, and create darkness: I make peace, and create evil: I the Lord do all these things.
Evil as a separate concept & entity became prevalent in Western religion based on the dualistic Zoroastrian model, where Good was represented by Ahura Mazda and Evil by Angra Mainyu. This was integrated into Judaism during the Persian occupation. It is from this period and its writings that we get Satan as the personification of evil, which found its way into Christianity and Islam: a powerful being working against the creator deity by tempting human souls into sin.
There are some similar myths in Buddhism, like Mara tempting the Buddha to prevent him achieving attainment, but by large the Karmic principle is not based on the forces of evil conspiring against humans, Buddhas and Bodhisattvas. Evil, both as a human characteristic and as an external force, is a foreign concept to Buddhism. Instead, each individual's choices and actions are the determinating factor in his/her Karmic destiny.
Which brings us to "The Problem of Evil": the question of how to reconcile the existence of evil and suffering with an omnipotent, omnibenevolent, and omniscient God.
In Theravada and some Mahayana schools, deities are irrelevant to Buddhism. It is personal practice and adherence to the path that results in attainment, not divine intervention. In Mahayana, this principle is known as 自力 (Jap: Jiriki), "one's own strength".
And yes, there are Mahayana schools that subscribe to 他力 (Jap: Tariki) meaning "other power", "outside help". Some believers of Pure Land Buddhism, for example, believe Amitabha Buddha will lead them to enlightenment, similar to Christians believing that Jesus takes away their sins to allow them into paradise. But even in Tariki schools, evil is not personified into an external, personalized force, but it does identify evil as part of our human nature that one must come to term with - in fact, this "negative thinking" is essential in Tariki schools to realize that one needs the help of a Buddha to reach attainment.
As for beings in hell, many Mahayana schools teach that every one of the Samsara realms has a Bodhisattva to assist those dwelling in that realm. For example, Ksitigarbha: Bodhisattva of the Hell Realm.