Evil is the opposite of good, so if we agree that actions to promote life, liberty and happiness are good, then actions that promote death, slavery and misery are evil. Simple as that.
So there's plenty of evil in nature.
All that matters in nature is whether something works or does not work, not whether it's good or evil.
Saying that, I have definitely met people with malicious intentions, who are definitely 'evil' (or lacking a severe amount of empathy for their fellow beings). I am not sure people with challenged sexual instincts are these people, however I am sure there are people with a combination of both behavioural traits.
Not a complete or concise definition but the general theme I notice when I think about what I personally consider "evil" is the elevation of your own desires over the safety or well-being of others. In a way, evil (for me) is based on what helps humanity/society or hurts it. It's based in our natural instincts for empathy and the ignoring of those "higher" motives in favor of more primitive drives to take, hurt, kill, rape, or enslave.
For me, good versus evil is not so much about religious or Hollywood ideas of light vs. dark or god vs. devil but the more familiar battle between our "animal" side and our "human" side.