Sin. Sins. Whatever.

For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord. - Romans 6.23

Why does God look at people's sins and not at all the good they've done?

So, a tour group is, say, at the Grand Canyon, and the day is hot. Fumes of heat rise from wilting yucca plants surrounding the sweating visitors. Their chapped lips are cracking. Their throats are parched. Their nostrils are, uh, dusty. They're on their hands and knees, gasping. Get the picture? Okay. The tour director stands in their midst with a tall, rotund pitcher of ice water. Clean, cold, drops of condensation trickle down the container's sides.
He asks, “Hey everybody, anyone want a drink?”
The tourists, slavering flakes of would-be saliva, crawl gasping inward.
Just as their hands clutch his pantlegs, the guy produces a vial of cloudy yellow liquid and an eyedropper.
“Violá,” he exclaims, “an anonymous urine sample!”
He siphons off a little, and plinks one putrid drop into the pitcher. He stirs.
“Now, who still wants a drink?”
The group backs away, their collective faces screwed up in grimaces of repulsion.
“Ah, c'mon,” the Tour Guide asks, “you're not gonna let a little pee-pee spoil all this refreshing water, are you?”

For me, it's a perfection thing. God's not unloving; he's pure, and demands purity. If heaven is going to be Heaven, then it's gotta be perfectly perfect. I, for one, don't want to spend my eternity on another planet earth.
...Great. I'm here in heaven for all eternity and I have to live next door to Charles Manson and Adolf Hitler just because they did some Nice Things in their lifetimes...

God's like a father who has to kick out a child who doesn't obey the rules of the house: He loves His children, but he has his standard, and his standard is perfection, not merely goodness. Sound harsh? Maybe. But He's head of the household. I'm content to let Him make the rules.