Garlic butter does eventually go bad.
Garlic butter should always remain refrigerated in an airtight sealed container when not using it.
When properly stored and kept refrigerated the garlic butter will last for as long as 15 days in the refrigerator.
After 15 days in the refrigerator the garlic butter can grow bacteria and mold and should be thrown out.
So make sure you use the garlic butter within 15 days so that it does not have a chance to go bad.
You can freeze garlic butter too so that it will keep longer than 15 days.
If you freeze garlic butter place the garlic butter in a Ziploc bag or freezer bag or other freezer safe airtight container.
When frozen the garlic butter will last for up to 6 months but after 6 months in the freezer the texture and taste of the garlic butter will usually change.