Hmmm... God and the Devil... Good and Bad! I'll keep this short! 😉 There is a God, simply, because we exist - He has created us and there will be no existence without Him.
For me, there is no such thing as the devil, simply evil people who believe that they are superior to God. Their vasana's ensure that they will never realise their ignorance. Man then says that evil is an inherent trait in him.
People need to believe in Karma to overcome evil. It is that every action has a reaction. If you know that in doing something wrong, wrong will happen to you - what is the purpose in doing something wrong?
Evil exists because man exists. We always want to test God, and his leash on us- not realising that we are testing ourselve, but falling very short of the "ideal". It started right at the beginning, when the apple was presented in the Garden of Evil oh sorry... Eden! When Ravan accosted Sitamata, when Duryodhan fought the Pandavas... and evil is represented righ to this day when a child is raped, someone is murdered, etc... different shades of evil, but its there.
NB: From what little I know, simply speaking a vasana is a habit or tendency -- usually operating compulsively and unconsciously. It limits our growth, responses and expressions. It is negative.