These are good questions for your pastor. I'm sure a smart one could give you some answers there. The best I can do is ask my Master, who knows Christianity pretty much inside and out and doesn't believe a word of it.

Here are my best guesses, since he's not home. Back in the old days, sacrifice solved a lot of problems, at least the people thought so. They were always sacrificing animals to try to make their harvest better/other animals thrive/children live. It is an effort to feel they had some control in a dangerous and unpredictable world. So having God make a sacrifice of his own son to make up for all people's sins should make people feel guilty enough to stop their sinning. No, we're not supposed to go on sinning. We're supposed to feel bad that we made God do such a terrible thing and shape up.