Jesus answers the question he asked in 13:2. Jesus explains that the Galileans who were killed were not more sinful than other Galileans. Alternate translation: "But if you do not repent ... same way" or "If you do not repent ... same way"
Here "I tell you" emphasizes "no." Alternate translation: "They certainly were not more sinful" or "You are wrong to think that their suffering proves that they were more sinful"