Speaking to the police, although normally not advisable, does not necessarily ruin the defendant’s chance of having their case dismissed or reduced. In some situations, talking to the police may help a case or avoid formal charges from being filed, but that’s not always the case.
I recommend always talking to an attorney before making this decision. There may be situations where they spoke to the police, and their statements are absolutely damning, but they are not admissible. This could be because Miranda Warnings were not read and those laws apply to your situation or because those statements should be suppressed for another reason.