About 30 years ago the WTB&TS took a hardline stance against members who held a job that required them to carry a weapon for the purpose of protecting themselves or others (I guess a gun carrying dog catch would be ok). The bottomline was any member who held such a job had 6 months to quit or they could not have any privileges in the congregation. As such, I do not believe it would prevent a person from being baptised, but once baptised they would not be able to do anything above field service and commenting at meetings.
The reasoning behind this was that it would put a person in a position that they might become "blood guilty". Remember, the WTB&TS does not differentiate between killing and murder. Even though the Bible specifically gives clearance from Jehovah to kill people to protect people and property.
I have an older cousin who spoke of a Witness in his congregation that was a police officer. He said this Witness was a very spiritual person, but would rather be a police officer than carry microphones at the meetings or be a Ministerial Servant. As a former Witness, former Ministerial Servant, and a 13 year law enforcement veteran, I don't blame him.
I also know of a man who was a sergeant at a sheriff's office whose wife and kids were all witnesses. He studied and got baptised. He didn't quit his job. He just voluntarily took a desk job inside the jail where weapons are prohibitted by policy.
In the police academy, during the deadly force instruction, our instructor handed out copies of Romans 13:1-7 from 4 different translations of the Bible. He said he went to the clergy of 3 different denominations of churches in his area and asked about those scriptures. He mentioned that as soon as he gave the passage the clergyman would all immediately say, "Oh, the police officer verse."
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New International Version (NIV)
Submission to Governing Authorities
13 Let everyone be subject to the governing authorities, for there is no authority except that which God has established. The authorities that exist have been established by God. 2 Consequently, whoever rebels against the authority is rebelling against what God has instituted, and those who do so will bring judgment on themselves. 3 For rulers hold no terror for those who do right, but for those who do wrong. Do you want to be free from fear of the one in authority? Then do what is right and you will be commended. 4 For the one in authority is God’s servant for your good. But if you do wrong, be afraid, for rulers do not bear the sword for no reason. They are God’s servants, agents of wrath to bring punishment on the wrongdoer. 5 Therefore, it is necessary to submit to the authorities, not only because of possible punishment but also as a matter of conscience.
6 This is also why you pay taxes, for the authorities are God’s servants, who give their full time to governing. 7 Give to everyone what you owe them: If you owe taxes, pay taxes; if revenue, then revenue; if respect, then respect; if honor, then honor.
The police are "God's servants" and when they use their "sword" they are doing the will of God.