Accoding to Bhagvad Gita --- Every soul is pure as it is part and parcel of GOD, but because of free will given to humans they want to enjoy senses and not want to serve GOD hence they became contaminated.
Every soul is not pure because we see all kinds of evil in this world.
According to Hindu scripture humans are contaminated because they do not want to serve God But the question arises what makes Krishna, Brahma and other hindu gods sinful. Something had contaminated them that's why they show sinful behavior like humans. Even they showed worst behavior than any normal human. If the factors of this world and sin has it's effect on them then they can't be god because it means they are not the controller of the universe but the universe (sin) is controlling them.