Hmm, so where were we? (Sorry I've been "out" for a while.)
The logically contradictory notion of the existence of a good God and evil sin.
This is the number one question for theologists to answer, and (IMO) a totally iron-clad answer is not to be found.
But...
A) God is not responsible for our sin:
"Let no one say when he is tempted, 'I am tempted by God'; for God cannot be tempted with evil and he himself tempts no one" (James 1:13).
B) Humans are responsible for our sin:
"Each person is tempted when he is lured and enticed by his own desire. Then desire when it has conceived gives birth to sin; and sin when it is full-grown brings forth death" (James 1:14-15).
C) The first, original (human) sin was committed by Adam and Eve. In its essence, it was a simple choice to rebel against God's simple rule (don't eat the forbidden fruit).
D) The existence of a truly free-willed creation necessitates the option to choose against God's will. Otherwise, we truly would be God's "puppets".
E) Finally, without the need to redeem us from our sin, there would be no vehicle for God to show His divine grace. (This is probably the crux of the matter)
Again, realize that I'm not trying to convince anybody; I'm merely offering an explanation of how God can be both perfectly loving and perfectly just. If ya got anymore questions, please post!