Jealousy and good and evil while finding meaning for life in Genesis 4:6-7

Today I have been teaching catechism (KT) and we tried to address issues of good and evil, as well as issues of commitment and engagement. I'll blog later about our reading of the whole of the passion narrative from Luke's gospel but we began by reading the story of Cain and Abel in Genesis 4. Where does evil come from, is it within us, beyond us? Verses 6 and 7 particularly struck me, full of profound theological insight. Here are two English translations for you to get a feel for:

The Lord said to Cain, ‘Why are you angry, and why has your countenance fallen? If you do well, will you not be accepted? And if you do not do well, sin is lurking at the door; its desire is for you, but you must master it.’

The LORD said to Cain:What's wrong with you? Why do you have such an angry look on your face?

If you had done the right thing, you would be smiling. But you did the wrong thing, and now sin is waiting to attack you like a lion. Sin wants to destroy you, but don't let it!

This part of the story spoke powerfully to me today about the anger vortex, how we can let ourselves be eaten up by it, how it can eat away at us and at our relationships. The French version we were working with today has sin spread like a carpet at the door ... interesting the different images used to get the message across.
Really though the question is how do we deal with our feelings of anger, of being mistreated. Will we let the feelings destroy us and others or will we try to take responsibility and address ourselves to a different kind of goal?