Chattymute, I'll try and do my best to answer all your questions. Again, i'm not trying to impose my beliefs - just want to share them...
Almost all your questions seem to revolve around evil and how it contradicts Christianity and yet I see you holding a double standard on the concept of evil.
Take for instance these 2 statements of yours -
(I'm assuming this first statement refers to the holy wars recorded in the Bible.)
God would despise evil, that doesn't mean you lower your standards and act "evil" in return (genocides).
Evil can't be defined. It is relative to that culture.
Your stance seems contradictory. If you believe in the second statement, then you shouldn't have a problem with the first - because perhaps in their culture, relatively speaking, genocide is not evil.
But that's not what you agree with, right? You seem to deny that evil can be defined and that it's definition is universal and absolute, yet you somehow seem to be applying your 'relative' definition of evil to complain against the justice in the holy wars.
So, pick one statement of the above two - if it's the 2nd statement, then you have no complaint against Christianity. We'll leave it at that then. But if it's the 1st statement, then let's continue discussing....
Now, even in your universal absolute concept of evil that you deny but seem to keep applying, you're mistaken about its definition. Evil as such does not have any definition independent of God. In fact no moral value has any definition apart from God. His laws too are not something that God Himself has to follow consciously - it's the other way around - it is what God wills by nature that He's laid out as laws.
So, if I wanted to grasp the concept of perfect love - I'd have to define it in God. If I wanted to grasp the concept of perfect good - I'd have to define it in God. And so on. Now, the absence of these attributes of God - even partial absence - is what gives way to imperfection. Absence of love(even partial) is hate. Absence of good(even partial) is evil.
So, God's nature and attributes define what is good and any violation against them is evil.
So, satan became a fallen angel because of his self-pride - the great sin. Self-pride and rebellion is what keeps man too away from God. Self-pride is where you think you can do without God - where you think you can do just as well - where you think your wisdom is greater than His - where you think your choices are better than His and so on. Basically, it's trying to free oneself from God's rule. Who would want to stay away from God? Who wouldn't want to obey God's will? Won't it be the people who are in enmity with God? And can enemies of God be anything but evil? Nonetheless, God provides salvation to us out of His mercy and grace.
So yes, it is possible to fall from a spiritual relationship with God as Adam did fall. God uses the fall to show us what we are apart from Him. Do you see the works of rebellious man under the rule of satan in this fallen world? Evil is our abuse of God's grace. Still, He makes known His mercies and points out our grave need for it. Those who are saved acknowledge wholeheartedly that they have been saved only by grace and not on any merit of theirs - and so they throw themselves to the mercies of God and deny selves. They offer themselves as living sacrifices to the Lord and rejoice in doing God's will alone. They are a new creation who know and experience the love, mercy and grace of God - such can and will never rebel against God and God chooses them to eternal life in the Kingdom of Heaven. This world is where we get to know God's mercies, judgement, wrath, righteousness and holiness through repentance, regeneration and sanctification. Here, God moulds us to be conformed to the image of His Son - Jesus Christ. God helps us learn to overcome the world, satan and our own sinful natures.
Is God both able and willing?
Then whence cometh evil?
Yes, God is both able and willing. evil comes from abuse of our freedom of will. To eradicate evil, God would have to eradicate us or teach us to obey His will. Now, He has chosen to teach us - but how many actually want to learn? On Judgement day, He will choose to eradicate evil once and for all from being present in the new world.
Seems to me he would want to forgive you. So why doesn't he give second chances after we die?
You've got your chance here - if you're not taking it now and here, then I doubt you'd take it any time later. Everything has been taught - all consequences have been warned of - absolutely your choice to ignore now and regret later.
It also creates the problem of why we even need to be on Earth. Since God is all-knowing, why do we need to live this life only to end up where he knows we will end up (all-knowing)?
The above paras should have answered this - we need to be on earth to learn to put our faith in God in total humility and to learn to obey His will.
A similar question that is put forth is why do we have to pray to God when He already knows what we are going to pray for. The reply is the same - prayer doesn't change God, it changes us - helps us learn to trust in Him more and more.
I was taught that sin is going against what God wants.
I don't see how sinning makes you evil.
You were taught that sin is going against God and anything against God is evil - hence sinning makes one evil.
Jesus wasn't much of a sacrifice. He died and went to Heaven like he would have done anyways.
The sacrifice was in coming into this world in the flesh when He could have remained seated in all glory in Heaven, facing suffering for our sakes, having sin imputed on Him even though He knew no sin, being offered as our guilt offering, being spiritually separated from His own Father, being crushed by the wrath of His own Father and facing death when He is the giver of all life.
This is the choice we had - His sacrifice or eternal condemnation for all. Of course, on the Last Day, those who are found to have treated His sacrifice with contempt do have to face judgement.
Why would an all-loving God send a other wise good non-Christian, or Christian, to Hell for eternity?
There is no 'otherwise good man' on earth. Of course, we can give our own standards of morals and deem most good but in God's sight, all fall short. Sinning is not the odd mistake or error here and there - it's a way of thinking in the heart. Either your heart is turned towards God or it isn't. If it's turned towards God, it means you are being sanctified by God through faith by His grace. If it isn't turned towards God, then you are necessarily evil. Just because God's common grace prevents us from letting loose our evil desires doesn't mean that we are 'good'. This is why we need Jesus as our only means of salvation. All people of other religions continue to believe in their gods because they love the freedom those gods give from the one true Living God - this is still rebellion against God. But if any of those are seekers of truth, God is faithful to reveal the truth of Christ to those earnest souls.
My point is - God looks into our hearts and sees every thought we entertain - a person might do a very good work of donating a huge sum of money to charity but might all the while be praising himself in his heart out of self-pride - this is evil in God's sight but good in man's sight.
Another eg: if a boy loves cookies and I ask him to stop eating them, he might rebel against me. He might stop eating while I'm there but might continue to eat when I'm gone. He might refrain from eating altogether but he might lust for that. He might wholeheartedly ignore cookies but turn towards chocolates that I've already asked him not to eat.
But in all these he has been wrong in his heart - there was no real loving obedience to my command.
You punish, and the end the punishment once a lesson has been learned. Getting a infinite amount of torture for a finite amount of sins is illogical.
You're right in that punishment is to lead one to repentance and to teach him to do good. God does that with all people here. But i think you're mixing up punishment and judgement/condemnation. The eternal lakes of fire is not punishment, it is judgement and condemnation for man's continual sinning and rebellion against God. Judgement comes only when the 'lessons of the punishments haven't been learned'.
Seems to me from what you described that God is just a dictator who essentially says " Do what I want to do because I say do it or you will go to Hell."
God is a Holy King who says " Do what I want you to do because it is the best for you or you will go to hell."
I hope this could provide some meaningful discussion...