Concerned about Atom

Recently, I was looking for good free text editors, and I came across Atom. After deciding that I preferred the first one I found, I uninstalled Atom. I only had one account on the computer at the time, but after uninstalling Atom, I created a second account for troubleshooting a screen saver problem (which seems to be a common one), and I found Atom installed on that new account. I'm pretty sure that it's not malware, because of the investigating I did before visiting the site, and the scanning of the installer and the computer I did afterward. Can anyone tell me why it reappeared in a new account, and how I can avoid that kind of thing?

Edited to add:
I left out the fact that I didn't delete Atom's installer right away and it may have still been sitting in the Downloads folder when I created that second account. I don't know whether I left out anything else which might be relevant, so if I did, you'll have to give me an idea of what kinds of things would be relevant. (Now, I'm pretty sure that things like account passwords are not relevant, so I'm probably going to ignore requests along those lines.:))

Edited to add:
I just discovered Atom listed on the Startup tab in the Task Manager. I disabled it, but it's still there after at least two restarts. I've had spyware which was less difficult to get rid of.

Resetting Windows got rid of that Startup task.