“Is it bad for a Christian guy to be a police officer? If yes in what way?”
The bible does not encourage it but there is no scriptural law or command that openly prohibits someone from working as a law enforcement officer. If you asked around, most Christians would much prefer that another Christian would be the police officer that they to deal with. If you are stranded alone on the road in the middle of the night, you too would prefer that the officer who finds you has strong Christian morals, would you not? I think the issue that some Christians have with working in law enforcement stems from the apparent discrepancy of bearing firearms with “turn the cheek” theology. In the Old Testament it was expected that every good Jew would do their part in defending Israel from enemies, whether foreign armies or local criminals. There are many Scriptures where God commanded his people to go to war or cause the death of offending parties (1 Samuel 15:3; Joshua 4:13, Exodus 21:12, 15; 22:19; Leviticus 20:11). Yet most if not all Christians find the New Testament teachings of Jesus about love and peace to be incompatible with causing the death of others.
There is an easy way to reconcile the situation, namely when we realize that a police officer is not called to kill but to save. In any uncorrupt society (so the Soviet Union does not apply) police officers are never called to indiscriminately kill others. And even if an event arises where the officer must fire his/her weapon to save lives, they always have the option of aiming at non lethal areas of the body to wound the attacker. The officer, like with all other careers, must make personal decisions about his/her morality. Provided you live in a country that doesn’t expect its police force to murder civilians, its permissible for a Christian to be a police officer. I personally know of a few evangelical Christians who work in law enforcement and are outstanding members of their local church.
As with all major life and career choices it is always best to consult with family as they will be most affected by your decision. Also many good Christ honoring congregations may likewise heavily disagree with your choice to merely wear a firearm, even if your intent is never to use it for lethal action, so it is best to know and understand your church. In all things Glorify God, if you are called to be a police officer, be morally upstanding and do your work to the safety of your community and the glory of God.
Please read Romans 13:1-7 for information about why law enforcement exists.