Fulfilling God's Purpose in Our Lives

Can we still fulfill God's purpose for our lives when we make poor decisions that are not part of God's will? What are we to make of Samson, a very unrighteous judge who God used to begin to deliver Israel from the Philistines? Since God apparently used his fleshly indulgences, are we ever justified to go ahead with sin in our own lives, such as marry an unbeliever in hopes they may turn to Christ?