I know a lot of you are tired of discussing Bellamy's actions this season but I want to offer the perspective of someone who just binged the first three seasons as my introduction to the show (thanks Netflix!).
I think the problem a lot of you have is that Bellamy's actions make no sense tactically, morally, intellectually. Totally fair. Not disputing that. However, I think they're also totally in character for him and I think they serve a more important thematic purpose. I find it perfectly realistic for him to backslide into being weak after his frankly quick turnaround to S2 hero as opposed to S1 asshole. For me it makes him a more real and complex character.
I also believe that the "point" of this storyline was to draw out the thematic and moral questions that come with the uncritical following of a strong, charismatic fascist leader (YMMV but I think that's what Pike is intended to be). I think this storyline has echoes of Hitler and the German people's complicity, and I think themes related to this come out through Bellamy's storyline: how far are we willing to follow in line with committing acts of atrocity.
I think Bellamy is perfectly suited to draw out these themes and storyline - we've seen that his character is a weak one, a follower, but a good soldier, easily manipulated (see the assassination of Jaha plot) and lacking moral guidance. He was also emotionally compromised at the time (Gina, whatever, and a perceived betrayal by Clarke, doubly so) and I think that allowed him to get swept up in Pike's rhetoric without being able to pause to critically think.
As the atrocities continue through the season, Bellamy becomes more and more entrenched in his belief that he's doing the right thing. To admit otherwise is to take on the weight of the countless murders he has already committed. I think it became an act of self preservation in order to stay sane and not be overcome by guilt.
Yes, a bit more of this could have been shown, I agree, but this show has consistently skimped on moments of important character development (for me, Finn's 180 degree personality swap is a case in point). I think it was perfectly clear otherwise.
Except for one thing... The fall-out/redemption... I do think it has been done very poorly so far. I guess you could say the characters have been caught up in the plot sweeping them along and haven't gotten time for much of a breather to think through the potential weight of all this... But it does seem like it's being filed away as just one more thing to live with... Which: no. This is definitely a step beyond what any of our other protagonists have done so far and needs to be addressed as such.
My hope is that the writers have seen the backlash and try to repair this. I think there's still time. Once the characters get some time to breathe and digest what has happened, the full moral weight will come down on them all and Bellamy will have to face some real consequences. I think this is still possible. Fingers crossed, because I actually thought this storyline was really interesting and I'm glad they did it. I really disliked Bellamy as a Mary Sue last season. I think his character has a lot of potential for interesting stuff, mostly because he is so weak (and yet very capable and charming, and the top billed male cast member - definitely a rare thing to see!).
Sorry that was so long but hopefully it made sense.