Hello
You can answer him this way:
Those who kill people are not true Muslims. Islam is a religion of peace,and as you already mentioned, it's forbidden to kill other people. For that reason Almighty Allah will punish them. Don't let terrorists spoil the real truth about Islam. Did you personally ask a Muslim if he or she is satisfied with those massacres? Be fair,don't judge Islam by the actions of a few. You need to read what Islam is really about and about the Prophet Mohammed peace be upon him. When you read the truth about Islam and the Prophet Mohammed peace be upon him, you will figure out that your first impressions were erroneous.
There is an English version of the Holy Coran, our holy book. Please read it and you will see.
Good luck