I think that it has been a constant throughout all of recent human history that refugees are looked at with suspicion or hatred. Being primarily tribal primates in our past it is only natural that we look upon outsiders with suspicion and do not want to share the benefits of our country with them.
Refugees are put under intense scrutiny and background checks, which is why stories of refugees fleeing often include guarding their "papers" with their life. There is almost no way that a terrorist could sneak in a s a refugee unless they became a terrorist later, and that argument could be made about anyone at any time.
Many people don't fully understand why people are fleeing Syria and cannot see why they put their families' lives at risk by paying people smugglers to help them escape. Heart-breaking images of lifeless children being washed up on European beaches make some people angry at the parents instead of directing their anger towards the Assad regime and those responsible for bombing Syrians out of their homes.
Syrian refugees get a bad rap. Most are non-violent, and are victims of the Syrian civil war. These poor refugees already suffered extensively when they lost their homes, as well as many family and friends. Furthermore, many refugees are often stigmatized by Islamic extremism: Westerners fear that Syrian refugees are members of ISIS.