Do I need additional security when using Windows 10?
Yes - absolutely! You cannot rely on software alone to protect your computer.
The viruses, trojans and rootkits of years ago make up an increasingly smaller percentage of attacks today, and they are mostly launched by amateurs. The largest percentage of attacks today, by far, are social engineering tricks that professionals use with great effectiveness. No antivirus software can defend against these attacks. Phishing attacks alone account for 90% of security breaches. In the cybersecurity profession we like to say "Criminals don't hack their way in any more - they simply log in."
As for software: By all means use an antivirus application. Choose whatever you like, whether Microsoft's own or a third-party product like Norton-branded software. You won't go wrong either way. But please don't think that antivirus software, from anybody, is enough to protect your computer. It's only a first step.