Do I need both antivirus (Intego) and antimalware (Malwarebytes) systems?

You do not need third party apps that claim to protect, cleamNmanage, boost performance, etc. Such apps include Avast, Sophos, AVG, Norton, Kaspersky, etc. The MacOS has built in protection. If you download malware it is often recommended here to use Malware Bytes to remove the malware. There are no virus's that affect MACS.

Semantecs aside, people see:

1. Launchdaemons from software they aren't expecting hijack their computer
2. Chrome policies rewritten.
3. Optimizers which delete LaunchServices caches, resulting in applications not launching without a wipe and reinstall, when they taught themselves as the best thing since sliced bread to keep your Mac speedy. When in reality, what is needed is two good backups, and not filling your machine to the kills with unnecessary bells and whistles. Many of them are advertised on top Mac news websites, download sites, and even exist on Apple's own App Store. When you hear people on Facebook who have downloaded the software, you just shrug, how do you prove to them, this stuff is not legit, when they blindly trust anything from the App Store as safe?
4. Mass email lists that get infected outside of the Mac platform, then spoof them to make them look like a spammer. The result being stuck in realtime blackhole lists and unable to email anyone.
5. Scary popups saying a machine is infected when it really is not, and the gullible person falls for the ploy to call a fake technical support line and give up their personal information, only to get their machine in more serious trouble. And yes, I've seen nefarious Launchdaemons stick the popup in startup so it keeps on recurring.
6. Websites that come over spam that look like real websites for the postal service and other commonly used services, when in fact they are redirects to download extensions in browsers that redirect the home page or worse.

Whether you call it a virus or a trojan horse, unwanted software does get on people's machines, and they can hurt performance, or make a machine inaccessible or hard to use.

Yes Macs have root account disabled by default. Yes registries can't get infected, when there are none to be had. But there are people who leave themselves open with no password and get fooled into downloading something which harms their machine.

To say there are no viruses is to lead one into a false sense of security.

One still need to be vigilant, and these malware detection tools do help.

Third party antivirus apps will not protect against trojans and malware.

Good article: https://discussions.apple.com/docs/DOC-8841

Intego has uninstallers. I would contact them for an uninstaller.

Malwarebytes and Avast are good and quite capable for manual malware removal.

Sophos isn't bad.

That said, this article can be helpful to identify viruses:

https://macreports.com/how-to-remove-weknow-ac-malware-macos/