I used EtreCheck and don't know how to fix what's RED

Do not get nervous. What is in red may not always reflect a serious problem.

What you should do is post the entire report and state what you believe to be the problem that prompted you to download the Etrecheck report.

Ciao.

Please describe the problem in as much relevant detail as possible. The "etrecheck" fad hasn't made that step any less necessary. The usual results of posting etrecheck output on this site without a full description of the problem are very poor.

The many "failed" or "killed" warnings that etrecheck sometimes spews are not a reason to reinstall the OS.

The better your description of the problem, the better the chance of a solution.

For example, if the computer is slow, which specific actions are slow? Is it slow all the time, or only sometimes? What changes did you make, if any, just before it became slow? Have you seen any alerts or error messages? Have you done anything to try to fix it? Most importantly, do you have a current backup of all data? If the answer to the last question is "no," back up now. Ask if you need guidance. Do nothing else until you have a backup.