Is there a term for someone who feels the need to constantly attack the belief systems or values of others?

Frightened.

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.

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

Threatened. They push others down to build themselves up.

Islamic Jihadist.

A troll perhaps. They just love to attack others.

Jackass comes to mind

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.

A Theological Right-Wing Bigot and that's goes no matter what religion you believe in.

A Rabid Pit-Bull Prosteletyzer who Knows Zero Boundaries and cannot be muzzled...

Ignorant. Pathetic. Insecure.

Ah, I change my mind on my answer.

I beleive that 'Chosen' works much, much better. :3

A fundie.

Actually there is a name for it..its discussed in the big book of rules(the bible) I cannot recall what the name is..but it does kinda say that you are not supposed to argue religion..btw ..if chosen one reads this..My father taught me that..He was theistic satanist..so..the devil knows the bible..backwards and forwards..and..that is something even true satanist do not do..:)

Egoist.

"Crisis of Faith" and "Denial" come to mind.

Lonely. (and for good reasons).

Jackass!

a proselyte or a missionary

1) "Proselytism is the practice of attempting to convert people to another opinion, usually another religion. The word proselytism is derived ultimately from the Greek language prefix 'pros' (towards) and the verb 'erchomai' (I come). Incidentally, "Proselytism is prohibited" in Greece, according to the constitution of Greece. Historically in the New Testament, the word proselyte denoted a person who had converted to the Jewish religion. Though the word proselytism was originally tied to Christianity, it is also used to refer to other religions' attempts to convert people to their beliefs or even any attempt to convert people to another point of view, religious or not. Today, the connotations of the word proselytism are often negative but this article will use the word neutrally to refer to any attempts to convert a person or people to another faith."
Source and further information:
http://en.wikipedia.org/wiki/Proselytism


2) "A missionary is a member of a religion who works to convert those who do not share the missionary's faith; someone who proselytizes. The word "mission" is derived from the Latin missionem (nom. missio), meaning "act of sending" or mitto, mittere, literally meaning "to send" or "to dispatch," the equivalent of the Greek-derived word "apostle" from apostolos, meaning "messenger". In Christian cultures the term is most commonly used for Christian missions, but it applies equally to any proselytizing creed or ideology. Buddhism launched 'the first large-scale missionary effort in the history of the world's religions'."
Source and further information:
http://en.wikipedia.org/wiki/Missionary

Further information:
http://en.wikipedia.org/wiki/Evangelism

There are a lot of terms for people who do that.
Coward
Arrogant
Narcissist
Mean
Fanatical
Etc...

I've heard it said that a Cynic is someone who knowa the price of everything, yet the value of nothing.

I suggest that "Cynic" would apply.

Nihilist

I don't know of the kind of attacks you have got. But if you are true believers, and hold values, you should thank him/her because arguments and information may let you grow (if you are really open minded). Turning the question a little bit upside down, wasn't through attacking the belief systems and values (if not plain massacres) of others how christianity and islam grew?

petty & insecure

A Dirtbag.

I think you are asking a good question. I think you and I both know that we see this happen.

What if we only see this because, we think that they are in oposition. There seems to be a "struggle".

You see, where there is harmony, in what is, and whats seen, then we know only wholeness.

You can't hit whats not there, and thats the psycological cycle that inrages, and exelerates, and tries to diminish and detroy. This comes from the "hardend heart of man".

God Loves... Aum...