Shinichi's behavior is pretty shit. He doesn't come off as a character, instead his only purpose is to serve as an emblem for the societal constructs of human weakness. It's all pretty heavy handed and lame because he doesn't have much of a personality of his own, and pretty much every moment he pipes up is dedicated to driving this point home instead of operating in an appropriate manner. Shinichi's ideologies aren't really given substance or merit. Because of this, it is easy to "hate" the way that Shinichi acts. There is no logic, there is no reason, and there aren't even hints of survival instinct. At the same time, he is not a particularly emotive character outside of reactionary shock.
See, I can agree with you that it is pretty heavy-handed. This series is pretty heavy-handed in general, and it wears its themes on its sleeves. That could make or break it with some people. I just don't really agree with him not acting in an appropriate manner. His character is pretty meek compared to being slightly goofy in the manga, but I think that'll work for this adaptation.
His ideologies aren't given much weight right now because it's his way of adapting; coping with having to coexist with an alien and with the knowledge of aliens that have been committing mass murders. I honestly think that the anime has done well in showing how he is trying to get to grips with the situation, and with the bombshell this episode that he might be merging it intensifies his growing rebellion against Migi, and it shows what he truly fears: losing the grips of his humanity. I'd say that if I have an alien for a hand I would try my best to constantly ascertain my own humanity. In his own way he feels that holding onto his humanity is the best way to combat the aliens.
There might not be much to his character other than to counterpoint that of migi but I think that works well considering the themes it explores.
I pretty much agree with this. It's very heavy handed and doesn't come across as natural at all. For a supposed weakling, he doesn't act like one yet he stutters and acts terrified half the time. Then the whole ending scene infuriated me to high heaven, I understand that he's shocked but the way that he acted just felt so unnatural. He's acting like he's in shock but he's not in shock at the same time.
As dimb said, his actions are almost completely contrary to his mindset.
Have you ever had to confront someone who is a bully and stronger than you? You feel like you have to do it, but it certainly doesn't lessen the fear you have, especially considering that the person will probably beat the shit out of you.
A weakling standing up to someone who is stronger, yet displays his fears, isn't contrary at all.