Sin is anything that is outside of God's design. Evil like murder is outside of God's design, as well as lying. God didn't make mankind with those purposes in mind. Anything outside of his design for human flourishing is sin. This includes omission as well as commission, which is not doing the right thing as well as doing the wrong thing. An example of omission would be not worshiping God as he ought to be worshipped.

The thing about sin, if you are a christian, is that it's forgiven by Jesus. Just because it is forgiven, doesn't mean you can just keep sinning. See the Lord's prayer where it mentions "..And forgive us our debts, as we also have forgiven our debtors...", meaning that our hearts will be changed and we can now forgive others, not that we earn our forgiveness by forgiving others.

Along with being forgiven, it is also not contagious. See how Jesus interacted with sinners in the gospels. This means you can be free to love sinners and share the gospel with your words and life. Remember that you were a sinner (defined by sin) and still sin, so separating yourself is probably not the right answer and most likely fueled by sinful motives.