A quick point first: humans are also the most helpful species on the planet. We're the only species which deliberately makes entirely altruistic donations to people and creatures we'll never meet or be connected to in any way. Even if there are exceptions (I know of none), we do it a lot more than any other creature.
You assume that without some external influence, humans would be entirely good. That means you probably believe that humans were created by a god, in a better state than they are now.
With those two assumptions, yes, you can presuppose some malevolent counter-influence such as original sin and/or Satan in order to make evil possible in a world created and run by an omnipresent, omniscient and omnipotent being. It's one possible solution to the age-old Problem of Evil, but one with its own pile of logical problems and without any evidence behind it.
On the other hand, you don't need these hypothetical influences to explain anything if you don't assume humans were created in the image of a god, or deliberately created at all.
If we have developed in response to our environment, being rewarded at different times by selfish acts and altruistic acts alike, and if we have risen above other animals in intelligence and ability to affect our environment, then without assuming anything else it makes perfect sense that we are collectively capable of and willing to commit the greatest acts of both what we call good and what we call evil. There is, in contrast to creation, mountains of evidence for this gradual development.
It seems like there's a disproportionate amount of evil in society because it's a worldwide and therefore HUGE society, and the news mostly jumps on negative stories. It's really a better world than that. Don't worry too much.
- SmartLX