Adrian was especially suspicious of Hilda after she took all his "special" items (items with "evil aura" like the poisoned tea) from his room. Hilda had acted like a disgustingly perverted loon to put Adrian off before, so this time he tried to confirm his suspicion by forcing her to drink the poisoned tea. For Adrian it's just a "test" to see whether Hilda can actually detect those "evil" things or if it's just a coincidence; hence no actual killing intent.
Btw, building countess' interest really saved her life. Which makes me more upset of the fact that MC missed adding the count into her interest list.
Awfully late to mention this now, but I'm really liking the artstyle; especially when it switches from detailed scary/serious scenes to the goofy and cute relieved/comedic scenes