i think D'Angelo's behavior in prison was completely justifiable and did not weaken his character at all
i think it's very human for a guy like him to withdraw into isolation after going through everything he did, and considering the life he lived, turning to drugs was a logical path to further get away from the presence of his family (something he couldn't even physically get away from with his uncle being in the same prison, exerting his influence)
that's what makes his death even more tragic, i believe
tried oh so very hard to leave that life behind but it catches up with him anyway
loved the scene in S3 where his mom confronts McNulty and he puts her down for being just as responsible for his death
and it's not even out of happiness to see her put in her place (well, there was a bit of that) but how emotional it was with her realization
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all you need