I thought it was too happy. I mean, it's good to see Sayu, Gotou, and Ayami all getting along. Maybe the harem route will work after all...?
That ending though is interesting.
I want to point something out. Yaguchi isn't mean or vindictive when he sees her. Obviously he doesn't pick up the fact she's terrified, but he definitely has trouble processing other people's emotions. He seems genuinely happy to see her, like meeting an old friend, or like he doesn't understand what their relationship was. It's not putting up a facade or trying to convince her.
That's why I think Yaguchi gets more shit than he deserves. I think he's just mentally ill or has some kind of disorder that makes him unable to really connect with people, like anti-social personality disorder. I don't think I can confidently call him a bad person or even really judge him too harshly because I genuinely think he doesn't understand what he was doing wrong.
I know it won't end well for him, but I kinda pity him, because I think if he just had a friend or someone who he could truly connect with and someone who he could rely on to work with him, then I think he could be a really good character or that he wouldn't have committed some of the things he did that were immoral.