I was rooting for 'sensei' from the start. The senpai was too clean and innocent for this kind of uke. But I do think they needed to develop the characters more. It came off as a 'blender manga'. Have all these traits, chuck them in a blender and see what happens.
Nothing was explored in enough depth. The tension regarding his mother didn't have time to build, he should have had more complicated feelings about the mistress. The teacher needed some sense of stability since he was all over the map. I also wasn't really feeling the love with Hikari for either love interest. His opinion of his senpai was like he wanted to keep him as an idol to look up to, pure and perfect. His opinion on his sensei was like sex = instant shift in affections. There was no feeling of confusion. We were told, not shown.
In general, I didn't mind it. I was happy Hikari ended up with sensei even if it was convoluted. It would have been worse if he ended up with his senpai.
Aijin Wa Korosareru