Victor Butler was a free agent the team signed from Dallas in 2013 because they were moving to a 4-3, the Saints were moving to a 3-4, and he’d be a significantly undersized 4-3 DE. He tore his ACL in his first Saints training camp then was released during his second one.
I’m not sure what the vision was for Khairi Fortt. I think they wanted to move him to 3-4 OLB, but he was injured during training camp then waived.
Arnoux was a 2009 draftee at linebacker. Another one for whom I’m not sure what the vision was, but he played middle linebacker in college and Jon Vilma had that spot locked down already, having been brought over in 2008.
I’m not a draft aficionado so I don’t like to do the “who could they have taken instead” game much. It’s just unfortunate that injuries they suffered cause them to not work out. Nobody gets it right 100% of the time.