Have you forgotten what happened to a team that Madrid played against before? I think you are taking this too lightly because even though City are ahead in this first leg, it is only 1 goal in aggregate and I hope you don't underestimate Madrid just because of this 1 goal aggregate.
I personally am still quite confident in the strength of Madrid and City must be prepared for that in the second leg later
So, I think Manchester City decided that the best defense is offense and they realized that containing Benzema is going to be really hard so they have to attack, and attack they did from the very start of the match.
It was a match with a lot of goals but in the end, there will have the advantage of only one goal. So, I think in the second leg Real Madrid will still have a great chance of winning because they do perform really great in big stages of football.