I have this long discussion with a Jehovah witness Colleague in my Office. He said their Church is the only one that doesn't support the killing of fellow Human as commanded by God-"Thou shall not kill". According to him , their members are not allowed to join the Army and therefore have no reason to kill.

Really, the Bible command that Christians should not kill. Is it then right for a Christian to join the Army and subject himself to roles that involve killing thereby breaking God's Command?

Well, the law (OT, Exodus 21:12-14) also differentiates between killing with malice or premeditated (murder) and killing by mistake, sans malice(manslaughter). The latter, being a violation of the commandment you quoted was punishable by death while the latter was resolved by the provision of cities of refuge.

Soldiers do not generally kill on the battle field due to a personal quarrel/ malice (I once saw an Afghan man taking care of an American soldier in Afghanistan) with other countries.

Rather the Army is one of the judgement-executing arm of the Executive arm of any Govt and also remember that Romans 13:4 clearly states that," For he (rulers: president, governor etc) is the minister of God to thee for good, but if thou do that which is evil. . .he (rulers) beareth not the sword (or gun) in vain, for he (rulers) is the minister of God, a revenger to execute wrath upon him that doeth evil" Besides the soldiers (being the sword bearing arm of the executive) exercises this power against external aggression or internal insurrection that threaten the life and well being of the territory God has set them over.

There are some unscriptural things we can cling to because of peace, how about in a war situation where that person is conscripted into the army by force by the govt?