Frightened.

Insecure. Someone who is content with his/her own beliefs does not need to belittle or attack others.

No. There is not just one word because people are different and have differing motivations:

Insecure, frightened, threatened, and jerk have all been suggested and all can be true.

My first thought was "insecure," because rather than stating and defending ones own beliefs, even contrasting them with others for the purpose of understanding, they feel that the argument need to end in their favor or they may lose what they believe.

Confidence in ones own beliefs can make us better witnesses for our own, but also can eliminate the need to be in attack mode.

There may be other motivations and I would become the attacker if I did not allow for these or for the possibility that I do not fully understand a person's motives. If I attack attackers I am become an attacker.

Some people feel so passionate about a good idea - especially after being convinced that the way they formerly thought was wrong that they believe it is their mission to bring others out of their old way of thinking. They are good hearted but immature.

Some people just lack some basic people skills. They have not learned how to communicate gracefully, to laugh at themselves, to be friendly, to disagree without being disagreeable, or to look for the best motives in others.

These skills can be learned.

If in the most appalling (to us) belief systems of others, no matter how basically antithetical to our own, there is always some good and there is always common ground.

Immaturity and lack of skill can be overcome. So can fear and lack of confidence, but it is a matter of choice.

We will always be more effective in promoting our own beliefs if our focus is not on tearing down those of others.

Good question.

Threatened. They push others down to build themselves up.

A troll perhaps. They just love to attack others.

Jackass comes to mind

Islamic Jihadist.

yes, it's Asshole.

Yeah ... jerk! Even if someone elses belief system IS wrong, you will NEVER get them to change by attacking them.

Mean.