You can't understand it, because you haven't accepted Jesus Christ as your savior. Once you do, it will all make sense. You say whatever you want, but traveling on a wide path of a highway which means doing what you think is right, gets you to Hell. Traveling on a narrow path which means following Jesus, grants you Heaven. We're all sinners, so getting mad at me won't do anything for ya. All I'm gonna say is you just don't know Jesus like I do.
In general, sin is just death. Indulge in it too long, and you just die. Smoke a joint, die from cancer for instance.
If Adam and Eve obeyed God, I have no clue what would happen. You can even ask someone more well-versed spiritually than I and they will say "I don't know" or make some guess. You'd have to ask God personally via accepting Jesus as your savior and follow him. Course you'd have to mean it body, mind, and soul. Otherwise, you'll just be a good moral guy doomed for hell.
What I barely know is heaven is a mansion. Jesus himself says in
John 14:1-6. It's the resting place if you've accepted Jesus Christ like a VIP ticket to be with the Lord. It's also a place with no suffering. I don't know anymore than that because the Lord is cryptic to me like a parent is to a child on the surprise. However, I know it's beautiful than all the things of this world if he promises us this place.
Hell is the place for those who don't accept Jesus. It's lake of fire that burns you for eternity and it won't cease. Home to people who really didn't want Jesus. Basically, you wouldn't want this resting place if you believed sin wasn't that bad. Link of what Hell is really like. Just read a few scriptures
here.
Regarding the smoking the joint example, if the person confessed with all his heart, mind, and soul he is a sinner and wants Jesus in his life, salvation awaits for him. Naturally he's gonna struggle with it, but Jesus will be his crutch when hurt. In the end, if he died of cancer but truly wanted to be with God doing his task that is to preach the gospel, then he'll make heaven his home.
If he rejects the gospel and remain in his sins however, he just goes to Hell. A place that would torture him for eternity.
As for the snake thing? Devil takes on many personas because he's clever like that. He could be anyone that tells you subtly not to serve God. He will possess and manifest someone(family, friend, etc.) to talk to you directly or give you thoughts that would please him and not God.
This is because the Devil aka Satan was once an angel who served God. His name was Lucifer. Because his pride got big due to how awesome he thinks he is, he went against God trying to be one. Then God took him down, Adam and Eve got tricked by devil, God sent Jesus so we can be saved, and God will finish it when he decides to. If you really want to know where pride came from, it was originated from the Devil.
We are sheep(Humans) for the wolf in sheep's clothing(Devil). That's why we need a Shepard(Lord) to guide us away from that wolf. Lest that lone prideful sheep will be destroyed by the wolf and become digested into poop. No one would want to end up like that.
Anyone could try to say God is evil, but where does it say he's evil in the bible? Or rather were any of us modern day peeps around when God made life? Truth be told, I myself have honest questions like that. However, God isn't evil if he set me free from certain sin issues I couldn't do myself. He also saved my mother from ending up in a coma and she's getting better(still has to work on walking from being bed-ridden). That in itself is enough to let me know he's beyond words to explain. Regardless, I'll try to answer as best I can.