There are many explanations throughout the Bible regarding Evil.   Why?   It is the nature of man to turn toward evil. We find an explanation is Proverbs that reinforces this fact! Proverbs Chapter 6  verses 12-18 

12)’A scoundrel, a villain, is he who deals in crooked talk.

13)He winks his eyes, shuffles his feet, makes signs with his fingers;

14)He has perversity in his heart, is always plotting evil, sows discord.

15)Therefore suddenly ruin comes upon him; in an instant he is crushed beyond cure.

16)There are six things the Lord hates, yes, seven are an abomination to him;

17)Haughty eyes, a lying tongue, and hands that shed innocent blood;

18)A heart that plots wicked schemes, feet that run swiftly to evil,

19)The false witness who utters lies, and he who sows discord among brothers.