As the end of the first season of Westworld draws near, it becomes more and more clear just how ruthless Ford truly is. At the end of Episode 7, viewers saw him use Bernard to kill Theresa and although Ford claimed that was the first time he'd asked Bernard to kill someone for him, that seems a bit hard to believe. What's also hard to believe is Arnold actually killed himself in the park. While others believe Arnold may still be alive, many are wondering if Ford killed Arnold on Westworld.
It's become evident that Ford will do whatever it takes to maintain control over his hosts and play God. He doesn't really care what Delos, the company that's supposedly running Westworld, wants to do. All he cares about are his creations and his narratives, and anyone who tries to get in his way is deemed a threat that must be taken care of. After it was revealed that Bernard isn't just Ford's right hand man but is actually one of Ford's hosts, one has to wonder how many people working at Westworld are actually hosts working for Ford's secret agenda. With that level of control it wouldn't be all that surprising if Ford did kill Arnold and made it look like a suicide.
After killing Theresa, Ford had Bernard throughly remove any proof that he killed Theresa, which included removing any evidence that proved they were more than just coworkers. Ford then had Bernard forget that he once loved Theresa and when Ashley even suggested that they had a relationship, Bernard truly had no idea what he was talking about. Although it seems Ford did a thorough job of erasing Bernard's guilt over killing Theresa in the preview for Episode 9, it looks like Bernard will begin to realize that Ford has lied to him again and Theresa may not have been the first person he killed. (RIP Elsie and potentially Arnold.)
Based on Ford's necessity for control over everything (and everyone) in Westworld, it would make sense that he killed Arnold. Ford would've wanted complete control over the hosts and Arnold — who wanted to give hosts consciousness — would've gotten in the way of that. If Bernard really is remembering his past murders and he really did kill Arnold, then the truth will hopefully be revealed soon.