I always felt like Riku was being toyed with an manipulated from the very beginning of KHI. He obviously talked to the robed figure, and was swayed into believing darkness was not a thing to be feared. After which, Malificent was completely filling him with lies as well.
In short; Riku had Terra's story before we knew Terra had Terra's story.
At least Riku did SOME penitance and has come out a 'stronger' person for it. Being trapped IN the Realm of Darkness, going through CO, taking on Ansem's SoD persona in order to use the darkness to protect Kairi/Sora, getting Roxas and Xion back into Sora so the latter could wake up. Terra's just possessed, he couldn't fight the darkness.
At the end of BBS, the same thing happens with Terra. Everyone he knows personally absolves him of any wrong doing and just kinda act as though he wasn't the sole reason all the shit that was happening happened. He ends up getting possessed as a result, and Aqua still sacrifices herself for him.
None of the characters of KH have any sort of regard for the consequences of their actions (the biggest culprit, in my opinion, being Sora) and this is a majour flaw in the entire series.