If you ask me for my personal opinion, antivirus is a waste for a smartphone.
It is enough with the security provided by the OS itself. Besides, nothing gets installed on your phone without your confirming it- unlike in PC. There are many points to support this logic:-
1. It has been estimated that less than 0.001% of apps installations evade Google's security measures. Even when there are thousands of malwares, there is no serious concern for Android users.
2. Google security has 7 layers of security measures:-
i) You get a unknown sources warning when you try to side load an app.
ii) You need to confirm installation. No app is installed without your confirmation. Will you confirm something that is suspicious?
iii) You need verify app permissions.
iv) Google conducts runtime security checks regularly to assess malicious behavior.
3. Some malware apps are just useless. When Google has a security in place, why pay for an app that practically does nothing is a waste of money and a waste on your resources like RAM, processor power, battery and storage space.
Those who root their phones and frequently download apps from 3rd party sources, yes- the threat exists. Even if you are rooted, and don't indulge in installing things from untrustworthy sources, you don't need an anti virus.
Finally, if you are really so scary of malware and virus, go for a free app. It will ease you of your worries, but will not be a burden on your pocket.