Is Islam really a religion of peace?

I would like to ask, is Islam really a religion of peace? My question arises because, why in some of your Quranic verses does Allah encourage you to kill specially those who do not believe in Islam? Quran (2:191-193 ) And Fight for the cause of Allah? Quran (2:244 ) And your God will kill those who do not accept this religion. Quran (3:56 ) I am a Catholic so to speak, and I made this question as a part of my research paper. Since I do not know any Muslim, I thought of asking it here. I ask for your wisdom, that is all.