re: Why would terrorist try to come over as a refugee?

It's more than just the issue of one terrorist finding his way past the gates of America.

ISIS and other radical groups are actively fomenting a global war. That's what they want. They say so repeatedly by their words and actions.

If subverting the humanitarian effort by Western democracies of taking in refugees will aid in their cause, and it would, then that is why they might choose to utilize the refugee migration to commit acts of terrorism.

Furthermore, the very ideology of Islam lends itself to radicalization. It's simple minded to think about Muslims as either 100% radical terrorists or 100% "moderates". The majority of Muslims, especially from war torn areas of the Middle East, support the goals of ISIS to some extent, at least philosophically, even as they resist the temptation to join in or support violence.

When you bring in Muslim refugees and add to or create new Muslim communities in a Western democracy, you are inviting problems. The cultures do not mix well. The Muslim communities often feel isolated and ostracized and lend themselves to added pressure to radicalize.

The desire to help, rooted as it is in a loving spirit, is misinterpreted by an active and motivated violent enemy as a sign of weakness and an opportunity to take advantage. That is just an unfortunate fact.

Western democracies will either be forced to compromise it's principles of acceptance for all ideogies and for freedom or they will be conquered from within or without. It's simply a matter of time and pressure. Time is running out and the pressure is very high right now.