Some are safe, some are not. The programs that are offered as test versions of a full-fledged and paid security tools are likely safe. Such applications have less functionality - no removal functions, no proactive protection, rare updates and so on. Free trial period upgrades it to a full version, but only for a while. However, that short time period gives the user a chance to have a look at the full functionality. After that, some basic security features remain, which makes it possible to use it as an anti-malware scanner. However, there are several antiviruses that declare having the full functionality for free. Those apps are considered as rogue software or scareware. Their only target is to make you pay them. Under the guise of “free malware remover” you will get the program that will randomly block the executive files of the apps you use, and ask for the “licence” to “remove the threats”. You will not be able to delete it or unblock your files - these apps cannot be deleted from usual interfaces. Fortunately, they are pretty rare nowadays.