Your question regarding whether Christians should be involved in the military or the police force where they are required to carry weapons is one that has been debated for many years by sincere Christians.
When our Lord Jesus walked the earth, he taught us by example and through the establishment of a commandment “to love the Lord thy God with all thy heart,” and to “love thy neighbor as thyself.” (Matthew 22:37-39) This new attitude toward even our enemies rejects the use of force and killing. “Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you.” (Luke 6:27-28)
As Christians we have our citizenship in heaven and are aliens or foreigners to any earthly government since we have already given our allegiance to the higher power—the Heavenly Lord. Although we are obedient to the laws of the land in which we live, we are not compelled to fight against our King. We are told, “submit yourselves to every ordinance” (1 Peter 2:13), except when our conscience, and God's laws, would be violated. Our Heavenly Father is looking at how we are living up to the teachings that were revealed to us by his dear Son. “We are therefore Christ's ambassadors, as though God were making his appeal through us. We implore you on Christ's behalf: Be reconciled to God.” (2 Corinthians 5:20) Let us then, “Follow peace with all men, and holiness, without which no man shall see the Lord.” (Hebrews 12:14)
Although we definitely believe that as Christians we should be conscientious objectors abstaining from military combat, whether we should choose an occupation such as a police officer is a little bit more nebulous. Police officers defend the populace from criminals and are most certainly needed to maintain order in our present society where Satan is prince. However, we must ask if it is wise for a true Christian to knowingly choose such a profession where he may one day be confronted with a situation where he will have to use his weapon and perhaps injure or kill a person, even an innocent person, in the line of duty. We would suggest if law enforcement appeals to you, that you choose an area within the profession where you do not have to use a weapon, such as an advisor, consultant or work in the many forensic fields available today. You will have the satisfaction of doing something positive for society without running the risk of violating Jesus’ commandment.