First off, I want to explore the idea of "karma". Typically this is seen as the "you get what you deserve," eye for an eye, tooth for a tooth kind of cosmic justice. This just seems wrong to me. Satisfying, yes, but it misses the whole point of life. Life isn't a giant account book where everything eventually evens out. Life is something our souls choose to experience so that it can learn, experience and grow. Our soul chooses the life we live. At every moment it decides what problems and challenges are going to confront us, so that it might experience those things. So karma is better seen not as what we deserve so much as exactly what we need.
But what about people who are born to abject misery? What about children who are born to sick, starving mothers just to die horribly? On the one hand, if you believe that every soul chooses it's destiny, you'd have to believe that that child's soul chose to be born at that place and time, to experience that particular life.
Just to take a step back, how about a less extreme example... You often meet people who seem stuck in their lives. From the outside, it's perfectly clear where they're going wrong, and either they can't see it themselves or they won't. These people are being offered the opportunity to grow but they always just run away from it because, let's face it, growth is painful. Is this "just their karma"? Or does personal choice come into play?
Everyone has a choice. Their choices may be more narrow or more broad, easier or more difficult, but we're constantly being presented with the ability to choose one thing or another, even if the only choice we can make is our attitude towards a situation. Accepting that the Ego is merely a useful construct helps us to detatch from it and take a larger view, but it doesn't negate personal responsibility and choice. In fact, our choice is everything. It's not what is done to us, but what we ourselves do that matters. But the more you detach from Ego, - the more you identify with the Soul behind the Ego - the less you can rely on rules. You have to feel out each situation and make the (usually) hard choice required.