Although the Bible doesn't speak of "good" Christians and "bad" Christians, it does acknowledge that sometimes believers can be disobedient and fall out of fellowship with God. (However, when someone repeatedly disobeys Christ, the Bible warns that they may not be a believer after all.)
I think, for example, of Peter. He was one of Jesus' closest disciples, and was the first to confess that Jesus was the Messiah, the divine Son of God sent from heaven to save us from our sins (see Matthew 16:13-17). But when Jesus was arrested, Peter fled (along with the other disciples) and later denied that he even knew Jesus (see Luke 22:54-62).
Was Peter a "bad" Christian then? Yes, he was, because he wasn't acting the way a follower of Jesus should act. And that can happen to us. But Peter's story doesn't end there, because he realized what he had done and sought God's forgiveness. And God did forgive him, and later Peter became a bold witness for Christ and one of the greatest Christians of all time.
Don't, however, use this as an excuse for being a lukewarm or halfhearted Christian. Jesus warned, "Because you are lukewarm - neither hot nor cold - I am about to spit you out of my mouth" (Revelation 3:16). Instead, make sure of your commitment to Christ, and then make it your goal to follow Him, no matter what it costs you.