When a Simple Greeting Led to a Threat

As a Christian, I deeply respect Muslims even though I don't follow Islam. At school, people have noticed my faith over time. Once, I greeted a Muslim classmate with "assalamu alaikum" and instead of hearing "wa alaikum assalam" or a similar reply, they threatened to harm me if I "disrespected" their religion again. I had no bad intentions-I just thought it was a proper way to greet them. Was this acceptable, or was I wrong?