Is NightLight Desktop a virus?
I'm not sure if you should be taking my word for it, but no. It's a legitimate tool I built to fill the void left by the old Icon Toolbox shutting down & to automatically upload scoreboard screenshots (coming soon) for those who use the Stats side of the website.
Thousands of people have installed and used it so far, and its development comes after 2 and a half years of working on nightlight.gg - a Dead by Daylight stats tool with more than 30,000 registered users.
If you have any concerns, I'd suggest running it through a tool like virustotal.com. If you're still concerned after that - there's manual ways of doing what the app does, I won't be offended if you don't use it ;)
Antivirus Issues with the Installer
Despite the thousands of users, NightLight is still a small project in the scheme of things. Antivirus software (understandably) errs on the side of caution with lesser seen downloads.
This does unfortunately mean that sometimes the NightLight Desktop installer will be flagged/blocked. It's much less common now that I've submitted some info to vendors but can still happen.
The warning you are most likely to see is from Microsoft's SmartScreen which isn't an antivirus but a reputation-based system by Microsoft. As it is reputation based, this warning should disappear over time.
I'm not entirely sure why but an old antivirus called Webroot really seems to have something against the installer (maybe for the reason below). There's a thread on r/antivirus about NightLight & Webroot which might help ease any concerns you have about this.
A More Technical Note
Like a lot of programs, NightLight Desktop uses an open source project called NSIS for its installer. Unfortunately, the installer is sometimes used by malicious programs as well. The unfortunate result is some antivirus programs wage a war against anything using it.