Finding old case records is difficult and many websites ask for a fee to access their records. You can find the general details of an old murder case by using a few different sources, possibly for free.
If used correctly, search engines are a great resource for finding details of old cases. Some search engines allow you to search their old news archives. If you have a name, location and year of the murder, you can find the reported details of the case. There is a chance the murder went unreported, but that is unlikely given the crime's severity.
Attorneys search case law when taking on clients to find rulings on similar cases. There are many websites dedicated to providing this service. They let you search old case records, and you can usually access recent cases without paying a fee. If you have the key details, you can pull up detailed legal information using these websites.
If you have a date, you can search your local newspaper's archives around the time of the murder. Some information may not be available online, so you may have to contact the newspaper directly to see if they have any records for the case you need.